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Ground-Fault Protection for Solar Applications
GROUND-FAULT PROTECTION
WHITEPAPER
Tyler Klassen, P. Eng.
Sales Engineering Manager
Littelfuse, Inc.
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Ground-Fault Protection for Solar Applications
Ground-Fault Protection or Commercial Solar PVSystems
Grid-connected commercial photovoltaic (PV) systems
are trending towards larger sizes, resulting in systems
with higher bus voltage and current levels. Problems
caused by ground aults are becoming a bigger concern
due to increased energy available at the point o ault;
arc-ash and shock hazards, equipment damage, and
fres can result.
When considering ground-ault protection, it is important
to understand the dierence between a grounded
and an ungrounded system. A grounded system has
one intentional connection rom either the positive or
negative bus to ground. Grounded PV systems are
commonly used in North America. An ungrounded
system has no intentional connections rom either bus to
ground. Ungrounded PV systems are commonly used in
Europe and Asia.
Grounded System
A grounded system has a single connection rom
one bus to ground, always located at the inverter.
This intentional ground connection is made througha use; the purpose o this use is to open when the
ungrounded bus aults to ground. A ground ault,
defned as an unintentional connection o an energized
conductor to ground, is a second path to ground and
completes a loop, causing ground-ault current to ow.
I the current exceeds the use rating, the use will open
the loop and stop the ground-ault current rom owing
When a ground ault occurs with current in excess o the
use rating, the use opens and the system becomes
ungrounded. However, the unintentional ground
connection (the ground ault) remains on the system unti
the equipment is repaired. A use monitor, such as an
indicating use and useholder are required to alert
Figure 1: Grounded Negative Bus
Figure 3: Up-LINK Indicating Fuseholder
Figure 2: Grounded Bus with Ground-Fault Current > Fuse Current
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maintenance personnel to the presence o the ault. The
Litteluse Up-LINK is a solid-state dead-ront useholder
thatprovides remote use-status indication without the
need or a proprietary network protocol. The Up-LINK
also eatures local LED indication.
Without ground-ault sensing and a disconnect
mechanism in the string combiner, the aulted cable will
remain connected to the system ater the frst ground
ault. The occurrence o a second ground ault, this time
on the unaulted bus, will cause a bus-ground-bus short,
potentially causing a very large magnitude o short-
circuit current, shock and arc-ash hazards, equipment
damage, and fre. It is important to detect ground aults
and deenergize the system in a coordinated manner
beore this hazardous condition can occur.I a ground ault has impedance, the ault impedance
will reduce the amount o ground-ault current. I the
ground-ault current is less than the use rating, the
use will not open. In addition, a ground ault on the
grounded bus will likely not cause enough current to
open the use. In an installation where wire resistance
is present, a voltage drop in the grounded-bus cable will
be present and may cause the use to open; however,
this is not guaranteed. I a ault on the grounded bus
does cause the use to open, the resulting voltage
across the open use will be very low and the use
monitor may not indicate that the use has opened.
Both o these examples illustrate a dangerous condition
that allows the ground-ault current to remain on the
system indefnitely. This can result in shock hazard,
equipment damage, and fre. The ability to detect
levels o ground-ault current below the use rating is
important or system saety.
The Litteluse Startco EL731 is a microprocessor-based
earth-leakage relay or grounded AC, DC, combined
AC/DC, and variable-requency power circuits. Earth-
leakage metering, and two setting levels (trip and alarm
are provided. In addition to ground-ault protection, the
Figure 4: Second Ground Fault ater Fuse Opens
Figure 6: EL731 Earth Leakage Relay
Figure 5: Grounded Bus with Ground Fault Current < Fuse Current
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EL731 has an input or a temperature sensor to provide
metering and overtemperature protection.
The EL731 uses sensitive EFCT-series current sensors to
detect as little as 30 mA o ground-ault current. Ground-
ault current sensing works on a core-balance principal;
with no ground ault, the current on the positive bus
will be equal in magnitude and opposite in polarity to
the current on the negative bus; the summation o these
currents is zero. When both the positive and negative
bus cables are passed through the window o a
core-balance current sensor, this summation is what the
current sensor reads. When a ground ault is present,
some current will ow external to the current-sensor
window in either the ground cable or some other path
through ground. The positive and negative bus currentswithin the current-sensor window will no longer add up
to zero; instead, they will add up to the amount o current
owing in the ground. A ground-ault current sensor can
also be used to measure current when a single ground
conductor is passed through the window. This method is
used to monitor the inverter ground connection through
the use. In an unaulted system, there will be no current
through this connection; during a ground ault, ground-
ault current will ow through this connection.
The EL731 can be applied to a grounded PV system to
detect ground-ault-current levels that are well below the
use rating, thereby lowering the levels o ground-aultdetection and creating a much saer system. An EL731 is
installed in the inverter, with the current sensor installed
to monitor the negative-ground path through the use.
In this location, an EL731 would detect a ground ault on
the ungrounded bus anywhere on the system. Since the
EL731 is much more sensitive than the grounding use, it
is also much more likely to detect a ground ault on the
grounded bus as well. However, the amount o ground-
ault current that ows during the ault will depend on the
resistance o the cables in the system, and may not be
enough to trip the EL731.
The EL731 output contacts can be connected to a trip
circuit used to isolate the inverter, or to an alarm circuit toalert maintenance personnel o the problem. The EL731
has LED trip indication on its aceplate, and optional
network communications can be used to remotely send
notifcation o a ground ault.
Ground-ault coordination is achieved by detecting a
ground ault, removing or isolating the minimum amount
o equipment required to clear the ault, and allowing the
rest o the system to saely remain energized. In addition
Figure 7: Grounded Bus with EL731 Earth-Leakage Relay
Figure 8: Grounded Bus with EL731 Earth-Leakage Relays in Inverter and String Combiner
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ground-ault coordination greatly simplifes location o
the ault or the maintenance team.
Proper ground-ault coordination uses time delays; relaysclosest to the system grounding point (inverter) are
set to trip slowest, and relays urther rom the system
grounding point are set to trip aster. I the frst relay
trips (the EL731 in the string combiner in the picture
above) and removes the ault beore the time delay o
the second relay (the EL731 in the inverter) expires, the
second relay will not trip and the rest o the system
will continue to operate. The maintenance team would
immediately be alerted to the presence o a ground ault
and would know which array o strings it is on by seeing
which relay is tripped.
To achieve ground-ault coordination over the entire
system, one EL731 and one contactor or breaker isinstalled in each combiner box. Each EL731 only detects
a ground ault in the array o strings connected to its
combiner box; it would not detect a ground ault in any
other array o strings. When a ground ault occurs in an
array, the corresponding EL731 will trip; the relays output
contacts will then trip a switch, breaker, or contactor to
remove the aulted array rom the system.
Figure 9: Grounded Bus with Coordinated Ground-Fault Protection
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An EL731 installed in the inverter is backup protection andalso protects the circuit between the string combiners and
the inverter. This EL731 is set with an extended trip delay
to give the EL731 in the string combiner the chance to trip
frst.
A current-sensing relay, such as the EL731, requires thesystem to remain grounded long enough to detect the
ault and to allow any programmed trip-time delays
to expire. Since the programmable trip-time delay
in the EL731 is 0 to 2 seconds, a ground-ault would
remain on the system or a maximum o 2 seconds. I
the use opens beore the relay trips, the ground-ault
current goes to zero, and the relay, no longer detecting
ground-ault current, will not trip. To guarantee ground-
ault coordination, a 5 A current-limiting resistor, R, is
Figure 10: Grounded Bus with Coordinated Ground-Fault Protection and Open Fuse
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An ungrounded system is defned as having no
intentional connection to ground. An ungrounded system
presents a problem or ground-ault detection. Since thebus has no intentional connection to ground, a ground
ault will not close a loop and ground-ault current will
not ow. Current-sensing ground-ault relays cannot
be used. It is worth noting that when the frst ground
ault occurs on an ungrounded system, the system now
becomes a grounded system through the ault until the
ault is repaired. This is not desirable as a subsequent
ground ault occurring on the unaulted bus will cause
a bus-ground-bus short circuit, resulting in short-circuit
currents, with possible arc-ash, fre, and shock hazards.
On an ungrounded system, insulation monitors are
typically used to measure bus-to-ground resistance.
With no ground ault, the monitor will measure a veryhigh value. When a ground-ault occurs, this value
decreases and the monitor responds accordingly. To
measure resistance to ground on a DC system, insulation
monitors inject an AC or pulsed DC signal onto the bus.
Insulation monitoring has two difculties; the frst is that
the capacitance o the system to ground presents a path
to ground to the insulation monitor, and i large enough,
will cause a nuisance trip. Since capacitance is a unction
o the PV system size, larger systems will have higher
capacitance and are more prone to nuisance trips.
The second problem is selective coordination is not
possible with insulation monitoring; an insulation
monitor will detect a ault anywhere on the system.Troubleshooting and ault location are difcult and time
consuming.
Resistance-Grounded SystemThe solution to these problems is to ground the system,
either by grounding the negative bus through the
methods described in the Grounded System section, or
by a more novel approach that approximates a popular
grounding technique used on AC systems. On an AC
system, the saest and most stable distribution system is
high-resistance grounded. A high-resistance-grounded
system uses a neutral-grounding resistor to connect
the neutral point o a wye (star) transormer secondary
to ground. The benefts o a high-resistance-groundedsystem are limiting ground-ault current to a low level,
elimination o transient overvoltages, elimination o arc
ash hazards caused by a ground ault, and the ability to
use selective coordination or ground-ault protection.
Many o these benefts can be achieved on a DC system
by grounding with a resistor network. In addition, a
ground ault on either bus can be reliably detected on a
resistance-grounded system.
Figure 11: Ungrounded Bus
Figure 12: Resistance-Grounded System
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To implement high-resistance grounding on a DC system,
a neutral or zero-voltage point must be frst established.
This can be achieved through the use o matchedresistors, R1 and R2. These two resistors are connected in
series between the positive and negative bus. The centre
point, S, is connected to ground through a PGR-2601 DC
Ground Fault Monitor.
The PGR-2601 is a microprocessor-based ground-ault
monitor or dc systems, designed to monitor a resistance-
grounded system.
The trip level o the ground-ault circuit is selectable rom
1 to 20 mA, and trip time is selectable rom 0.05 to 2.5 s.
When used with the PGR-2601, the resistor network is
designed to limit the ground-ault current to 25 mA, and
to allow a nominal unaulted current o 12.5 mA. The totalrequired resistance between the positive and negative bus
is calculated using Ohms law; RTotal=V/I. For example,
on a 1,000-Vdc bus, the total required resistance is RTotal
= V/I = 1,000/0.0125 = 80 k. When there is no ground
ault on the system, the voltage at the centre point o this
resistor network, S, is 0 V, and no current ows through
the S-to-relay-to-ground conductor. When a ground ault
occurs on either bus, the voltage across one resistor
rises to ull bus voltage, causing 25 mA o current to ow
through it, the S-to-ground connection, and the PGR-2601.
The PGR-2601 detects this current and trips.
To achieve proper ground-ault detection and coordinatio
or both buses, a resistor network can be used along with
an EL731 and breaker or contactor at each combiner box.
Figure 13: Resistance-Grounded System with Ground Fault
Figure 15: Resistance-Grounded System with Coordinated Ground-Fault Protection
Figure 14: PGR-2601 DC Ground Fault Monitor
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ConclusionA grounded system is superior to an ungrounded system
as it allows the use o current-sensing ground-ault relays
to quickly and accurately detect low levels o ground-ault
current. A system with one bus grounded through a useand current-limiting resistor allows ground-ault protection
or the ungrounded bus; however, ground-ault detection
and protection or the grounded bus is not guaranteed.
A system that is resistance grounded through a resistor
network is the best solution, as it allows ground-ault
detection and protection or both buses. Relays and
disconnect mechanisms are installed at each combiner box
to detect a ground ault and remove the minimum amount
o equipment to clear the ault. Not only is this system the
saest, it is the most convenient to maintain and will reduce
system down time.
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