IEEE Electrification Magazine - December 2013 - 43

Current (A)

Bus Voltage (V)

systems and provide a reliable way to coordinate fault
R(Ω)
protection to isolate the smallest portion of the system
D
neighboring a fault. many of these relays can be easily
R1-..
adopted to work in a corresponding dc distribution sysC
R1-4
tem powered by sources with small internal impedances
No-Trip Zone
and are thus capable of providing fault currents much
B
larger than the nominal system currents. in the case of dc
R1-3
A
R
distribution systems powered by sources interfaced to the
1-2
Trip Zone
distribution bus through current-limited power converters, some of the traditional detection methods used in ac
are not appropriate because fault currents limited by con10
20
30
40
50
60 t (ms)
verters are not much larger than normal operating curFigure 6. Resistance-time trip characteristic: the trip characteristic for
rents. in fact, the methods based on current threshold and
contactors that tie the main distribution bus. The resistive threshold and
current derivative threshold cannot be effective because
time threshold can be designed depending on the system configuration.
of the similarity between nominal operation currents and
short-circuit fault currents. the methods based on voltage
sags (minimum voltage) measurement might be good for
gives very detailed information about the state of the bus,
tripping the converters offline when a fault happens but
this method requires high bandwidth measurements and
they cannot discriminate between faults in different locaintense computing capability and may therefore be more
tions. for this reason, the coordination of converters and
suitable for identifying a fault location after the system
contactors can be difficult. the methods based on impedhas been protected rather than for detecting a fault to
ance or distance relays are widely used in traditional distake initial action.
tribution and transmission systems with a radial
We propose a new approach based on time evolution of
configuration and the presence of multiple sources in difapparent resistance as the discriminating characteristic for
ferent locations. in fact, if there are too many radial lines
a short-circuit fault. this method requires much less comand busses, the time delay for breakers closest to the
puting capability and is more suitable for fast protection.
source becomes excessive if the protection system is
Under the assumption that the faulted path has a much
based on current-time tripping
curves. instead, relays that respond
to a voltage-current ratio are more
6,000
sensitive to faults than those that
respond only to current. in case of a
5,000
fault, the impedance relay sees the
4,000
impedance shifting from a domi3,000
nantly resistive impedance to a
2,000
Conv 1
smaller and dominantly inductive
Conv 2
1,000
Load
impedance that is typical of the line
0
impedance. in this way, the imped0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
ance relay permits discrimination
Time (s)
(a)
between faults in different locations.
Fault
at
Load
Fault on Main Bus
Whereas in ac systems the relay
Load
Connections
reacts to changing impedance, in dc
7,000
systems direct measurement of a
Conv 1
6,000
complex impedance is not possible
Conv 2
5,000
Load
because there is no fundamental
4,000
frequency on which to base the
3,000
notion of impedance. some emerg2,000
ing methods propose to measure the
1,000
impedance spectrum of the bus by
0
injecting a broad-spectrum current
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Time (s)
perturbation and then measuring
(b)
the resulting voltage perturbation
and extracting the associated
impedance magnitude and phase Figure 7. Fault detection and coordination: (a) voltage and (b) current on the main bus during
a short circuit fault close to the load (t = 0.3 s to t = 0.34 s) and during a fault on the main
spectrum. even though such a mea- bus (t = 0.5 s to t = 0.56 s). Normal operation transient events do not trip the fault protection
(t = 0 s to t = 0.25 s).
surement of the bus impedance

IEEE Electrific ation Magazine / d ec em be r 2 0 1 3

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