Dynamic Li-ion energy storage boosts power grid stability

An artist’s impression of the DynaPeaQ system Source: ABB
Combining energy storage systems with fast-acting grid interface electronics opens up a new route to dynamic control of active as well as reactive power.

Michael Lippert, Saft, France, and Marguerite Holmberg, ABB, Sweden

The increasing penetration of intermittent renewable energy sources is a significant source of instability for transmission and distribution grids already operating close to their limits, especially in periods of peak demand.

On-line, spinning generators loaded at less than full output have traditionally been used to address frequency regulation, a primary concern in the event of a sudden, unexpected loss of power generation or transmission. But an energy storage system (ESS) operating in combination with fast-acting grid interface electronics offers a new solution that enables dynamic control of active as well as reactive power in a power system, independently of each other.

Through 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.

The first dynamic energy storage solution for grid applications to become commercially available is ABB’s DynaPeaQ system, which combines SVC Light technology with Saft’s advanced Li-ion battery modules. DynaPeaQ has been developed to provide 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 is a power electronic converter used as a reactive power compensator and an established element of the group of technologies known collectively as FACTS (Flexible AC Transmission Systems). With a STATCOM, the reactive power absorbed/injected can be varied continuously. In addition, the speed of response of a STATCOM is higher than traditional SVC systems, enabling it to counteract much faster variations, so as to ensure voltage stability by varying the reactive power absorbed/injected. A STATCOM utilizes 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 SVC Light with energy storage, the DynaPeaQ system can control both reactive power ‘Q’ ” operating as an ordinary SVC Light ” as well as active power ‘P’ ” see Figure 1.

Figure 1: ABB’s DynaPeaQ system combines SVC Light technology with Saft’s Li-ion battery modules Source: ABB

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 ESS can then provide the necessary burst of active power to maintain stability and later be recharged from the grid during normal conditions.

Li-ion battery system

Since the DynaPeaQ is designed for high-power applications, and series-connected IGBTs are used to adapt the voltage level, the pole-to-pole voltage is as high as 10 kV. Therefore, a number of battery modules must be connected in series to build up 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 many features:

  • high energy density;
  • very short response time;
  • high power capability 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 3000 cycles at a depth of discharge of 80 per cent or 1 million cycles at a depth of discharge of 3 per cent. The ESS 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. In a design study for a 5 MW ESS system, the battery was designed for an energy capacity of 5 MWh, in order to supply 5 MW of power for 30 minutes ” equal to 2.5 MWh ” from a 50 per cent state of charge.

Saft medium-power fully-aged Li-ion cells have a discharge power of 146 W per cell. Thus, for an energy capacity of 5 MWh, the minimum number of battery cells required is 34 247.

In order to have identical battery strings, as well as a DC voltage suitable for the DC-link of the converter and to meet the current required for the discharge time, the actual installed number of battery cells is 36 288, divided into two battery strings. With this optimal design, the actual installed energy capacity of the battery system is 5.36 MWh.

Battery system energy rating

It is possible to design an ESS that provides only the minimum amount of energy for dedicated operation in regulation service. However, such a single-use system can have disadvantages. From a grid-operation standpoint, a storage system with limited energy requires almost symmetrical charge and discharge cycles in order to prevent the storage from becoming too full or too empty. Symmetrical charge and discharge cycles, however, are not a realistic assumption taking into account the variable and unpredictable nature of power flows in a grid. In most cases, additional energy is therefore necessary to enable the ESS to operate in a certain bandwidth of state of charge.

From a battery standpoint, limiting the energy rating has two main impacts. First, each charge-discharge cycle will represent a larger percentage of the installed energy, causing the available cycle life to be used up more quickly. Second, the power-to-energy ratio for charging may be too high to be compatible with a long battery life. Charging at very high rates can result in a significant level of ageing in all Li-ion batteries with graphite negative electrodes. The acceptable charge power is a function of the battery’s performance type (e.g. high power, medium power or high energy) and of the installed energy.

Another factor is that ESS schemes can be more easily justified if they can address multiple value streams. For example, additional battery energy can be used to provide synchronized reserves. Sizing the battery system to supply 5 MW for 30 minutes, and using it to supply à‚±5 MW of regulation service, will have the following benefits:

  • Regulation and reserve services can be supplied by the same system;
  • Average cycle depth is reduced, extending the operating life;
  • The one-hour discharge time allows use of a more cost-effective medium-power battery option (rather than high-power), while still providing 5 MW of charge power without serious life impact.

Power-frequency characteristics

Figure 2 illustrates the system frequency profile (left) and power-frequency characteristic of the primary frequency controller (right). When the frequency is within the deadband, the power from the battery is zero. When the frequency moves outside the deadband, the battery will charge or discharge as necessary. So if the frequency exceeds the upper limit, the battery charges (absorbing excess power from the grid). If the frequency falls below the lower limit, then the battery discharges (injecting active power into the grid).

Figure 2: The system frequency profile (left) alongside the power-frequency characteristic of the primary frequency controller (right) Source: Saft

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 SOC level of the battery storage is set at 50 per cent.

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. It focuses on safety and ensures the capability to respond to the consequences of possible faults. In addition, the solution offers low losses and very high cycle efficiency. The VSC is built up of power IGBTs and diode semiconductors. To handle the required valve voltage, the semiconductors are connected in series. Water cooling is utilized for the VSC, resulting in 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 multiple semiconductor chips (ABB StakPakà¢â€ž¢ semiconductors).

DynaPeaQ 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 on-line 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 is also able to provide power quality control in conjunction with railway electrification, as well as to help balance power in wind and solar generation schemes, which are inherently intermittent.

In 2010, a pilot system was installed on an 11kV radial distribution network operated by UK Power Networks (previously EDF Energy Networks) in the east of England. 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 will be 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 has been provided by the DNO 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 is 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 that the two feeders have quite different characteristics. On the first, the most significant demand occurs during the night time, due to the high number of homes that are heated by night storage heaters. This also means that 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 that events requiring grid support are likely to occur at different times, maximizing the utilization of this pilot system. Operational testing of the pilot DynaPeaQ installation has now commenced.

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