Page 168 - Decision Making Applications in Modern Power Systems

132 Decision Making Applications in Modern Power Systems

Generally, the studies are done in the time-varying simulation modes, con-
trolled by control elements, the StorageController.
The control element performs tasks of energy charging dispatches in a
controlled manner, and some loading and unloading modes can be found as
follows and in the program [20]:
PeakShave: Storage starts unloading when the monitored load exceeds an
established value of active power. Thus it has attempted to maintain
demand within a range.
Time: In this mode, charging and discharging start from a preset time.
Follow: This mode also tries to keep the load within a traditional value,
but, unlike the PeakShave mode, the charging and discharging start from
a time to be set.
Load shape: Both charge and discharge follow an established curve, so
when the values are negative, the storage unit is charging, and when the
values are positive, the storage is in the discharge mode.

5.3.5 The load model
The load model in OpenDSS is a PC element that can be defined by its nom-
inal power (kW, kVAr, kVA) and PF. It can be modified by load multipliers,
daily or yearly load shapes, for example. By default, the load behaves as a
current source, and its primitive Y matrix contains the impedance between
the neutral of the load to the ground if existent [14]. However, for fault stud-
ies, it would be convenient to change the default behavior of the load to
admittance, thus including it in the system Y matrix.
The load models can be characterized in different ways. There are eight
options to define how the loads will vary with the voltage:

Constant real and reactive powers ðP 1 jQÞ. This is the default option of
the software, and it is widely used for power flow studies.
Constant impedance ðZÞ load.
Constant real power PðÞ and quadradic reactive power QðÞ. Similar
behavior of a motor load.
Exponential. By default, it is a real linear power PðÞ and a quadratic reactive
power QðÞ. Used for feeder mix studies or voltage optimization measures.
Constant current magnitude IðÞ: It may be used to represent rectified
loads.
Constant real power PðÞ and fixed reactive power QðÞ.
Constant real power PðÞ and fixed reactive impedance.
ZIP model. Array of eight coefficients for weighting factors and cutoff
voltage.
In any case, new considerations should be done for load modeling on har-
monic analysis. The harmonic load model in OpenDSS is a Norton