IEEE Electrification Magazine - September 2015 - 48

example, in October 2013, the California Public Utilities
Commission established a target of 1,325 MW of energy
storage for the Pacific Gas and Electric, Southern California
Edison, and San Diego Gas and Electric companies by 2020,
with installations to occur no later than the end of 2024
(http://docs.cpuc.ca.gov/PublishedDocs/Published/G000/
M079/K533/79533378.PDF). One of the technologies that is
being deployed is battery energy storage. As such, an ad
hoc group within the WECC REMTF recently took on the
task to quickly develop a simple battery energy-storage
system (BESS) model for power-system stability studies.
The results of that effort are presented here.

Utility Installations of Battery Energy Storage
BESSs deployed for power-system applications range from
several hundred kilowatts to many megawatts. In the majority of the applications today, the technology used is that of
lithium-ion (Li-ion) solid-state batteries. In this type of battery,
lithium ions move from the negative to the positive electrode
during discharging to produce an electric current to inject
into the grid and then back from the positive to the negative
electrode during charging when the current is coming from
the grid. Li-ion batteries present some advantages over traditionally used lead-acid batteries in that they are much lighter
and thus have a significantly higher power density and can
typically produce higher voltages. There are many different
types of Li-ion batteries, with varying battery chemistry and
performance; a detailed discussion of such aspects is beyond
the scope of this article. Typically, Li-ion batteries have a relatively long cycle life and minimal self-discharging rate such
that most Li-ion batteries will discharge at a rate of between 1
and 2% per month if left standing and disconnected.
The complete BESS installation for power-system
applications usually consists of three main parts:
xx
The energy-storage module is usually made up of
numerous battery cells connected in parallel and series
to constitute a single, large energy-storage module. For
example, a single battery cell might have a rating of
12 Vdc and 1 kWh of energy. By connecting, e.g., 100 cells
in series and ten such units in parallel, one can create

Figure 1. A 1-MW, 2-MWh battery (foreground) and its 1.25-MVA converter system. This system was applied for smoothing and regulation at a
10-MW wind plant on an island. (Photo courtesy of WEICan.)

I E E E E l e c t r i f i cati o n M agaz ine / SEPTEMBER 2015

an energy-storage module capable of a maximum of
1,200 Vdc and 1 MWh. Figure 2 shows the racks of energy-storage modules for an actual BESS installation in
North America.
xx
The power converter is a power electronic device built
using high-power insulated-gate bipolar transistors
(IGBTs), which allows for controller switching of the power
electronic circuit to convert dc produced by the batteries
to ac, at the required nominal frequency, for injection into
the bulk electric power grid. For North America, the ac
voltage needs to be at a frequency of 60 Hz.
xx
Controls are associated with the power converter, typically implemented as software uploaded into a digital
control system. The controls manage all aspects of the
BESS, from charging to discharging, and services such
as voltage regulation, frequency regulation, and controlling variability of wind or PV generation colocated
with the BESS, etc.
The setup for a BESS is shown in Figure 3. The power
converter is similar to the power converters used in many
other applications, such as the grid interface of wind turbine generation and PVs. The power converter, using
IGBTs, allows for what is known as full four-quadrant control-this is shown in Figure 4. That is, within the current
rating of the power electronics, the converter is able to
independently control the active and reactive current
being injected into (or absorbed from) the power system.
This type of power converter is called a voltage source
converter (VSC).

Capabilities of Battery Energy Storage
The VSC interface between the energy-storage modules
(the batteries) and the power system give the BESS tremendous flexibility. The VSC is used as a grid interface in
many applications, including wind turbine generators, PV
generation, and even advanced pumped storage hydrogeneration. As described previously and shown in Figure 4,
VSC allows for full four-quadrant control, i.e., the converter is able to take the dc voltage of the battery and independently, within the current ratings of the converter,

Figure 2. Energy-/power-storage modules inside a commercially operational
BESS in North America. (Photo courtesy of Electric Power Research Institute.)

http://docs.cpuc.ca.gov/PublishedDocs/Published/G000/

Table of Contents for the Digital Edition of IEEE Electrification Magazine - September 2015