02-11-2012, 06:01 PM
NEW DIRECTIONAL GROUND-FAULT ELEMENTS IMPROVE SENSITIVITY IN
UNGROUNDED AND COMPENSATED NETWORKS
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ABSTRACT
This paper introduces two new directional elements for determining ground fault direction in
ungrounded and compensated distribution networks. These elements only require information
from the protected feeder and provide very high sensitivity. First, we review the basic
characteristics of these distribution networks. Next we analyze the steady-state operation of a
ungrounded and compensated system in the phase- and symmetrical-component domains under
ground fault conditions and describe existing methods for detecting ground faults in these
systems. We conclude by introducing new fault detection methods for compensated systems and
show how those methods are implemented with modern relay technology.
INTRODUCTION
Ungrounded systems have no intentional ground. For a ground fault on these systems, the only
path for ground current to flow is through the distributed line-to-ground capacitance of the
surrounding system and of the two remaining un-faulted phases of the faulted circuit.
In resonant-grounded or compensated distribution networks the system is grounded through a
variable impedance reactor, which compensates the system phase-to-ground capacitance. The
reactor, known as the Petersen coil, permits adjustment of the inductance value to preserve the
tuning condition of the system for different network topologies.
Because ground faults in ungrounded and compensated systems do not affect the phase-to-phase
voltage triangle, it is then possible to continue operating either system in the faulted condition.
The system must have a phase-to-phase insulation level and all loads must be connected
phase-to-phase.
Resonant grounding provides self-extinction of the arc in overhead lines for about 80 percent of
temporary ground faults [1]. Considering that about 80 percent of ground faults are temporary
faults, we conclude that more than 60 percent of ground faults in overhead lines clear without
breaker tripping. Ground faults represent more than 50 percent of all faults in overhead lines.
Ground relays for these systems require high relay sensitivity because the fault current is very
low compared to solidly grounded systems. Most ground-fault detection methods use
fundamental-frequency voltage and current components. The wattmetric method [2], [3] is a
common directional element solution, but its sensitivity is limited to fault resistances no higher
than a few kilohms. Other fundamental-frequency methods (such as the admittance method
[4], [5]), provide increased sensitivity but require information about all feeders and/or the
possibility of making control actions on the Petersen coil. There are also methods that use the
steady-state harmonic content of current and voltage to detect ground faults [6], [7]. Another
group of methods detects the fault-generated transient components of voltage and current [6], [8].
These methods have limited sensitivity, because high-resistance faults reduce the level of the
steady-state harmonics and damp the transient components of voltage and current.
GROUNDING METHODS OF MEDIUM-VOLTAGE DISTRIBUTION NETWORKS
The main goals of system grounding are to minimize equipment voltage and thermal stresses,
provide personnel safety, reduce communication system interference, and give assistance in rapid
detection and elimination of ground faults.
With the exception of voltage stress, operating a system as ungrounded or resonant grounded
restricts ground fault current magnitudes and achieves most of the goals listed above. The
drawback of these grounding methods is that they also create fault detection (protection)
sensitivity problems. We can create a system grounding that reduces voltage stress at the cost of
large fault current magnitudes. However, in such a system the faulted circuit must be
de-energized immediately to avoid thermal stress, communication channel interference, and
human safety hazards. The disadvantage is that service must be interrupted even for temporary
faults.
UNGROUNDED DISTRIBUTION SYSTEM ANALYSIS
In this section we analyze the steady-state behavior of ungrounded systems in both the phase and
the symmetrical component domains.
Three-Phase Analysis
Figure 3 shows a simplified representation of a three-phase ungrounded distribution system. The
relay location defines the protected line. All the other distribution lines are lumped in an
equivalent line representing the remainder of the distribution system. For simplification in our
steady-state analysis, we assume ideal sources operating at nominal frequency, and no load, and
disregard line series impedances (resistance and reactance). We justify disregarding load on the
basis that all loads for these systems must be connected phase-to-phase and thereby do not
generate any zero-sequence unbalance. These assumptions introduce no significant error in the
results but greatly simplify the calculations.
In Figure 3, CAL, CBL, and CCL represent the phase-to-ground capacitances of the protected line,
and CAS, CBS, and CCS are the phase-to-ground capacitances of the remaining network. We do not
represent the phase-to-phase capacitances of the system in Figure 3 because they do not
contribute to the residual current and are irrelevant to this analysis.