IEEE Power & Energy Magazine - May/June 2021 - 72

Directional overcurrent typically uses phase-angle
differences between voltage and current to
determine the direction.
Limitations of Conventional, Nondirectional
Overcurrent Protection
Nondirectional overcurrent protection trips when current
levels are higher than normal. This is the most frequently
used protection method in distribution systems and behindthe-meter utilization systems. Nondirectional overcurrent
protection devices typically include fuses and molded-case
circuit breakers. Distribution switchgear and reclosers are
also typically set to trip on nondirectional overcurrent. This
protection strategy activates protection only when fault
current levels significantly exceed the normal operating
current. Fault-current levels in a microgrid can vary significantly when the system is in grid-tied or islanded operation.
DERs can also impact fault-current levels, which make conventional, nondirectional overcurrent protection difficult to
use in a microgrid of this nature.

N
North Bay Community
Energy Park

E
S

Fraser St

Chippewa Creek

Kinsmen Way

YMCA

Thomson
Park
North Bay
Memorial
Gardens
Sports
Arena

Kinsmen Way

Chippewa St

Community
Energy
Park

figure 1. A map of the North Bay Hydro Community
Energy Park microgrid area.
72

ieee power & energy magazine

At the CEP microgrid, fault-current levels while islanded
are significantly lower than fault-current levels while grid
tied. When the microgrid's transformers are initially energized, they draw higher-than-normal currents to magnetize
the transformer core in a phenomenon called transformer
magnetizing inrush. With a strong source, transformer magnetizing inrush can exceed 10 times the normal full-load
current. Normal current levels during transformer magnetizing inrush while grid tied could exceed fault-current levels while islanded. This means that overcurrent protection
settings that reliably detect faults while islanded may not
be secure while grid tied, and overcurrent protection settings secure while grid tied may not reliably detect faults
while islanded.
Furthermore, directional overcurrent protection is needed to
selectively coordinate protection. In contrast to conventional,
nondirectional overcurrent protection, which considers only
the current magnitude, directional overcurrent considers current and voltage phase angles to determine the direction of
the fault-current flow. For example, consider a scenario of
islanded operation where both of the CEP microgrid's CHP
generators are running. Protection at each generator must be
directional to determine whether the fault is on the protected
generator or elsewhere in the microgrid, a fact that cannot be
determined by current magnitude alone.
Directional overcurrent protection in microgrids is also
a challenge. Directional overcurrent typically uses phaseangle differences between voltage and current to determine
the direction. In microgrids, faults can cause the system frequency to vary, sometimes rapidly. Changes in frequency
can challenge phase-angle estimation.

Power-Import Minimization and
Export Restrictions
Many electrical utility bills are structured to include a
demand charge based on the maximum power demanded
or imported by the site loads and an energy charge based
on the total energy consumed or imported. To reduce the
site's electrical utility costs, the CEP microgrid is configured to minimize both peak demand and the total energy
imported. At this site, power export (on-site power production exceeding site load demands) is allowed for only a
limited duration.
To meet both the power-import objective and export
constraint, DERs are dispatched within the microgrid
to maintain a low level of import. However, with a lowimport level, the microgrid may be incorrectly prompted
may/june 2021

IEEE Power & Energy Magazine - May/June 2021

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - May/June 2021