IEEE Electrification Magazine - December 2013 - 44

smaller impedance than the unfaulted load paths, the
faulted path impedance dominates the definition of the
equivalent impedance at the terminals of any particular
protection device. the voltage-current ratio calculated at
the terminals of converters and contactors is determined
by the resistance of the cable between the device and the
fault. since the equivalent resistance seen by any one contactor will be different depending on the position of the
fault, this allows a protection algorithm to differentiate
between faults close to or far from
the device and to take appropriate
action. this provides a fast and
effective method for detecting and
identifying faults, and importantly, it
relies only on local measurements.

Protection coordination

A new approach has
been proposed for
the protection and
rapid reconfiguration
of such systems,
capitalizing on
the inherent
controllability and
current-limiting
capabilities of
electronic power
converters.

a fault discrimination algorithm
based on apparent resistance can
be implemented in any converter
that is capable of measuring voltage and current at its terminals.
the ratio is then used by the converter decision algorithm to give a
current-limiting reference or an
enabling signal. When the converter recognizes an equivalent resistance lower than a predefined
threshold, it goes into current-limiting mode, bringing the current
down to some predetermined
small value. the converter stays in
this mode until the equivalent
resistance returns above some predefined threshold after
a predefined time delay. in this way, the hysteresis of
the algorithm avoids oscillations between fault and nonfault conditions.
a similar fault detection algorithm is present in the
controller of each contactor that segments the dc bus and
in each contactor between the dc bus and the loads. in
these contactors, the decision-making algorithm has multiple statuses to provide coordination between contactors.
since the protection scheme relies on fast-acting contactors, not circuit breakers, and since contactors cannot
open against fault currents, each contactor is allowed to
open only after its own current falls below its rated opening current. at this condition, a contactor can open the
remaining current without damage. for converters the
fault discrimination resistance has a single value, for

contactors the threshold has a time characteristic set after
exceeding a trigger value, together with a current direction
condition that allows contactors connected at the same
node to distinguish between different directions of the
fault current. the threshold values are defined by a resistance-time trip characteristic that can be properly
designed for each contactor, as shown in figure 6. When
the measured equivalent resistance falls below the resistance-time threshold after the alarm condition, and the
current is positive, the contactor is
responsible for opening. the defined
time steps of the resistance-time curve
can be determined by a number of
things including the distribution bus
time constant and the control bandwidth of the connected power converters. the resistive thresholds, instead,
can be selected depending on the distribution bus configuration and the
parameters of each cable section
between devices. to provide selectivity
and redundancy between contactors, a
time interval between one threshold
and the next threshold can be defined
depending on the time constant of the
distribution bus and on the time to
open the contactor.
simulation results demonstrate the
effectiveness of the fault detection
method and the coordination between
feeding power converters and mechanical contactors for fault protection in a
multiterminal medium-voltage dc distribution system. a particular study system operates at a
rated voltage of 5 kv and a rated power of 40 mW and has
two power sources interfaced to the dc bus through controllable power converters. as illustrated from the plot of
the current in figure 7, fault currents can be very similar
to current dynamics due to different kinds of load connections-load step (t = 0.05 s), a step of a constant power
load (t = 0.15 s), a load connection with a capacitor
(t = 0.22 s), and a short-circuit fault (t = 0.3 s and
t = 0.5 s) . however, the equivalent resistance measured at
the terminals of the two feeding converters shows a clear
distinction between fault conditions and dynamics due to
normal operation.
When a fault happens (t = 0.3 s and t = 0.5 s), the converters are briefly de-energized, the voltage collapses for a
short interval, and then the bus is re-energized as soon as

I E E E E l e c t r i f i c ati o n M agaz ine / december 2013

Table of Contents for the Digital Edition of IEEE Electrification Magazine - December 2013