IEEE Electrification Magazine - September 2015 - 49

As many utilities are beginning to see BESSs being proposed and introduced into their systems, particularly in
the western U.S. system, there is a need to model this
technology in a simple yet effective way for large-scale
power-system stability studies.
In recent years, one of the key challenges with renewable generation technologies, such as wind and PV generation, has been the development of standard public computer simulations models for power-system studies. These
public-domain and standard model structures, which do
not specifically pertain to any vendor's equipment, are
referred to as generic models. The concept is that the model
structure is flexible enough that through proper model
parameterization, the model can reasonably emulate the
dynamic behavior of a large range of different vendors'
equipment. A large effort to this effect was embarked upon
several years ago in WECC, which culminated in the development of the second generation of renewable energy system models that can be used to model both wind and PV
generation. The model specifications are publicly available
on the WECC website, and have been validated with measured field data from several different wind turbine types.
These second-generation models were developed with a
focus on modularity so that a set of modules were developed, each representing an aspect of the renewable energy
system, and thus, each specific plant can be represented by
connecting together the right combination of modules, e.g.,
the generator/converter model plus electrical controls models plus the plant-wide controller, etc. Thus, the approach
taken in developing the BESS model was to augment one of

Power
Converter

Energy-Storage Modules

dc Power

Power Transformer/
Switch Gear

Figure 3. The parts of a BESS.

the modules to allow for modeling a BESS while using all
the other modules that have already been developed and
equally applicable to a BESS installation, such as the converter model, the plant controller, etc.
One key point needs to be made before further discussion of the BESS model. The generic stability models
developed for renewable energy systems are intended for
large-scale power-system simulations, which are typically
performed using simplified stability models. These models
have their limitations, and so, one needs to clearly appreciate that they are not suited to every type of analysis and
simulation work. This is true of any model.

Reactive Current
Converter Current Limit

Quadrant 1:
-ve Active Current
+ve Reactive Current

Quadrant 2:
+ve Active Current
+ve Reactive Current

Quadrant 3:
Quadrant 4:
-ve Active Current
+ve Active Current
-ve Reactive Current -ve Reactive Current

Active Current

Modeling of Battery Energy Storage
for Stability Studies

ac Power

To Bulk Electric Power
System

control the real and reactive power being injected into (or
absorbed from) the ac side, which is the bulk electric
power system. In the case of BESS, since the batteries are
rechargeable, real power can be either injected (discharging) or absorbed (charging). Furthermore, because the
IGBTs are power electronic switches that can be switched
at kilohertz, the converter can go from discharging to
charging in a fraction of a second, resulting in a tremendously flexible and functional BESS. A BESS, if controlled
properly, can provide the following services:
xx
voltage control and regulation at the substation where
it is connected in the power system
xx
frequency support by either providing very fast frequency response or being part of the automatic generation control (or secondary frequency response)
xx
power oscillation damping, if placed in a strategic
place in the power system and complemented with
the proper supplemental controls, or
xx
help to reduce the net variability of variable generation sources if combined with or colocated with, variable generation facilities such as wind or PVs.
There are other potential applications that will likely be
identified as the technology continues to mature.

Charging = -ve Active Current
Discharging = +ve Active Current
Boosting Grid Voltage = +ve Reactive Current
Reducing Grid Voltage = -ve Reactive Current
Figure 4. Four-quadrant operation of the VSC interface between the
energy-storage modules and the grid.

IEEE Electrific ation Magazine / S EP T EM BE R 2 0 1 5

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