Littelfuse Startco SE-330HV, Руководство пользователя

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SE-330HV Neutral-Grounding-Resistor Monitor
Rev. 5-M-101817
1. G
ENERAL
1.1 M
ODERN
R
ESISTANCE
-G
ROUNDED
S
YSTEMS
A high-resistance-grounded system uses a neutral-
grounding resistor (NGR) with a low let-through current
to limit ground-fault current. This is an improvement
over low-resistance or solidly-grounded systems
because, in those systems, a ground-fault flash hazard
exists and a ground fault can result in substantial point-
of-fault damage. High-resistance grounding eliminates
these problems and modern ground-fault protection
operates reliably at low current levels. Furthermore, the
probability of an arc-flash incident is significantly
reduced in a high-resistance-grounded system.
NGR selection depends on system charging current
and whether the system is an alarm-only or a tripping
system. Alarm-only systems are usually restricted to
system voltages up to 5 kV with NGR let-through
currents of 5 A or less. Occasionally, alarm-only
systems up to 15 kV and up to 10 A are used; however,
they are not common because a ground fault on such a
system tends to escalate to a phase-to-phase fault before
the ground fault can be located and cleared. Consult
CEC 10-1102, NEC 250.36, and NEC 250.186 for
application details.
System charging current is the capacitive current that
flows to ground when a bolted ground fault occurs. This
current can be calculated or measured. For small
systems, the magnitude of charging current can be
conservatively estimated as
½ A per 1,000 kVA on low-
voltage systems and 1 A per 1,000 kVA on medium-
voltage systems.
In an alarm-only system or in a tripping system
without selective coordination, choose an NGR with a
let-through current larger than the system charging
current and set the pick-up current of ground-fault
devices at or below 50% of the NGR let-through current.
In a tripping system with selective coordination, use
ground-fault devices with a definite-time characteristic to
achieve time coordination. Use the same pick-up current
for all ground-fault devices—this value must be larger
than the charging current of the largest feeder. Select an
NGR with a let-through current between five and 10
times the pick-up current of the ground-fault devices.
Do not use a grounding transformer with a low-
voltage resistor:
•
The combined cost of a transformer and a low-
voltage resistor is more than the cost of a resistor
rated for line-to-neutral voltage.
•
A transformer saturated by a ground fault through a
rectifier
can make ground-fault protection
inoperative.
•
Transformer inrush current up to 12 times rated
current can cause a ground-fault voltage larger than
expected.
•
A parallel transformer winding makes it difficult to
monitor NGR continuity.
•
A transformer can provide the inductance necessary
to cause ferroresonance if the NGR opens.
Following these guidelines will reduce the flash
hazard, reduce point-of-fault damage, achieve reliable
ground-fault protection, and ensure a stable system not
subject to ferroresonance.
1.2 SE-330HV NGR M
ONITORING
The SE-330HV is a microprocessor-based neutral-
grounding-resistor monitor that detects NGR failures and
ground faults in resistance-grounded systems. The
SE-330HV measures NGR resistance, NGR current, and
transformer or generator neutral-to-ground voltage. The
components required to monitor an NGR are an
SE-330HV, a 100- or 200-k
W
ER-series sensing resistor,
and a current transformer (CT).
Power-circuit elements, other than neutral-connected
NGR’s, that purposefully connect the power system to
ground are often not compatible with SE-330HV NGR
monitoring. These elements include single-phase
grounding transformers, grounded-wye-primary potential
transformers (PT’s), and grounded-wye-primary power
transformers.
The
SE-330HV continuously measures NGR
resistance in an unfaulted system. It will trip on resistor
fault if NGR resistance varies from its calibrated value.
When a ground fault occurs, voltage is present on the
neutral and NGR current will flow if the NGR is healthy.
The SE-330HV will trip on ground fault if fault current
exceeds the GF TRIP LEVEL setting for an interval
equal to the GF TRIP TIME setting. However, if the
NGR fails open during a ground fault, it is possible for
fault resistance to satisfy the NGR resistance
measurement. To detect this double-fault condition, the
SE-330HV measures neutral voltage. If neutral voltage
exceeds the V
N
TRIP LEVEL setting, and if NGR current
is less than 5% of the current transformer (CT) rating, the
SE-330HV will trip on resistor fault. If the resistor-fault
circuit is tripped and the neutral voltage exceeds the V
N
TRIP LEVEL setting for an interval greater than the GF
TRIP TIME setting, the ground-fault circuit will also
trip.
Ground-fault current is sensed by a CT with a 1- or 5-
A secondary, or by a CT (EFCT-x or SE-CS30-x) with a
50-mA secondary. The trip level of the ground-fault
circuit is adjustable from 2 to 100% of the CT rating and
trip time is adjustable from 0.1 to 10.0 seconds.
The SE-330HV has four output relays. Relay K1 is
the trip relay. Relays K2 and K3 provide ground-fault
and resistor-fault indication. K4 is a solid-state relay that
provides UNIT HEALTHY indication. Relay K1 will
operate on either a resistor fault or a ground fault, and it
can be set to operate in the fail-safe or non-fail-safe
mode for undervoltage or shunt-trip applications.