IEEE Power & Energy Magazine - May/June 2019 - 80

The main challenge is to achieve the desired reliability,
in terms of security and dependability, within a
very small amount of time.

refined using signal processing methods to improve their
function in a noisy environment. communicationless protection algorithms, based on undervoltage and overcurrent, are
more suited to complement main or backup protection. the
respective undervoltage or overcurrent criteria have a fixed
time requirement. in contrast with the fastest algorithms,
these two criteria do not rely on the shape of the waveforms
during the fault transient.
communication-based protection algorithms combine
information from both line ends to make a tripping decision and, therefore, require a communication channel to
exchange logic signals or measured quantities from both line
ends, as shown in figure 6(b). the communication channel must allow for high-speed communication and most
likely would rely on fiber optic cables. the algorithms can
be categorized as those communicating a status (directional
comparison) and actual measurement data (differential). the
protection zones are specific for communication-based protection algorithms and do not need dc line inductors to separate the protection zones.
A directional comparison can be implemented as either a
tripping or a blocking scheme. in both cases, a fault in the forward direction is detected at the local line end, which needs to
be confirmed by information from
the remote line end. in a tripping
scheme, the presence of the fault is
Dampened Transient,
Sharp Transient,
More Difficult Detection
Easy Detection
confirmed by a tripping signal from
the remote line end. A tripping
Protection Zone: Cable
signal is issued by the remote line
end if the measurements at that end
(Remote)
(Local)
indicate a fault in the forward direcdc Line
dc Line
Protection
Protection
tion, i.e., in the direction of the protection zone. in a blocking scheme,
(a)
the presence of a fault is confirmed
Smallest
Largest
by the absence of a blocking signal
Communication Delay
Communication Delay
from the remote line end. A blocking signal would indicate detection
Protection Zone: Cable
of a fault in the reverse direction
at the remote end of the protection
(Remote)
(Local)
zone. the tripping scheme introdc Line
dc Line
Protection
Protection
duces the largest number of delays
Communication
in a directional comparison scheme;
these delays occur at the local end
(b)
of the protection zone. While the
fault is perceived almost instantafigure 6. A comparison of protection algorithms (a) without and (b) with communineously by the local measurements,
cation. (Note that the comments refer to local dc line protection.)

based only on local measurements or on the communication
of measurements from distinct locations in the grid.
Protection algorithms without communications rely only
on locally measured voltages and currents, as shown in figure 6(a). these algorithms can be classified as those detecting a fault during a transient and those monitoring a signal
for a certain amount of time.
Many of the proposed algorithms without communications detect faults during the transient following the fault
event. Among these are voltage-derivative, current-derivative, combinations of voltage- and current-derivative,
and traveling-wave algorithms. the algorithms determine
whether the fault is inside the protection zone by using the
shape of the wavefront following a fault. Differentiating
wave shapes for internal and external faults requires knowing the electrical distance. this electrical distance is typically driven by the series inductors required for the HVdc
circuit breakers, which act as low-pass filters on the wavefronts generated by external faults. remote faults on long
cables are more difficult to detect than closer faults because
of wavefront distortion and attenuation during propagation
on the transmission line. the basic fault-detection methods based on transient voltages and currents can be further

80

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may/june 2019
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