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GROUND-FAULT PROTECTION
FOR COMMERCIAL SOLAR PV
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 faults are becoming a
bigger concern due to increased energy
available at the point of fault; arc-flash
and shock hazards, equipment damage,
and fires can result.
When considering ground-fault
protection, it is important to
understand the difference between a
grounded and an ungrounded system. A
grounded system has one intentional
connection from either the positive or
negative bus to ground. Grounded PV
systems are commonly used in North
America. An ungrounded system has no
intentional connections from either bus
to ground. Ungrounded PV systems are
commonly used in Europe and Asia.
GROUNDED SYSTEM
A grounded system has a single
connection from one bus to ground,
always located at the inverter. This
intentional ground connection is made
through a fuse; the purpose of this fuse
is to open when the ungrounded bus
faults to ground. A ground fault, defined
as an unintentional connection of an
energized conductor to ground, is a
second path to ground and completes a
loop, causing ground-fault current to
flow. If the current exceeds the fuse
rating, the fuse will open the loop and
stop the ground-fault current from
flowing.
When a ground fault occurs with
current in excess of the fuse rating, the
fuse opens and the system becomes
ungrounded. However, the
unintentional ground connection (the
ground fault) remains on the system
until the equipment is repaired. A fuse
monitor, such as an indicating fuse and
fuseholder are required to alert
maintenance personnel to the presence
of the fault.
Without ground-fault sensing and a
disconnect mechanism in the string
combiner, the faulted cable will remain
connected to the system after the first
ground fault. The occurrence of a
second ground fault, this time on the
unfaulted bus, will cause a bus-groundbus
short, potentially causing a very
large magnitude of short-circuit
current, shock and arc flash hazards,
equipment damage, and fire. It is
important to detect ground faults and
de-energize the system in a coordinated
manner before this hazardous condition
can occur. If a ground fault has
impedance, the fault impedance will
reduce the amount of ground-fault
current.
If the ground fault current is less than
the fuse rating, the fuse will not open. In
addition, a ground fault on the
grounded bus will likely not cause
enough current to open the fuse. In an
installation where wire resistance is
present, a voltage drop in the groundedbus
cable will be present and may cause
the fuse to open; however, this is not
guaranteed. If a fault on the grounded
bus does cause the fuse to open, the
resulting voltage across the open fuse
will be very low and the fuse monitor
may not indicate that the fuse has
opened. Both of these examples
illustrate a dangerous condition that
allows the ground-fault current to
remain on the system indefinitely. This
can result in shock hazard, equipment
damage, and fire. The ability to detect
levels of ground-fault current below the
fuse rating is important for system
safety.
The Littelfuse Startco EL731 is a
microprocessor-based earth-leakage
relay for grounded AC, DC, combined
AC/DC and variable-frequency power
circuits. Earth leakage metering, and
two setting levels (trip and alarm) are
provided. In addition to ground-fault
protection, the EL731 has an input for a
temperature sensor to provide metering
and over-temperature protection.
The EL731 uses sensitive EFCT-series
current sensors to detect as little as
30mA of ground-fault current. Ground
fault current sensing works on a corebalance
principal; with no ground fault,
the current on the positive bus will be
equal in magnitude and opposite in
polarity to the current on the negative
bus; the summation of these currents is zero. When both the positive and
negative bus cables are passed through
the window of a core-balance current
sensor, this summation is what the
current sensor reads. When a ground
fault is present, some current will flow
external to the current-sensor window
in either the ground cable or some other
path through ground.
The positive and negative bus
currents within the current sensor
window will no longer add up to zero;
instead, they will add up to the amount
of current flowing in the ground. A
ground-fault 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 fuse. In an unfaulted
system, there will be no current through
this connection; during a ground fault,
ground-fault current will flow through
this connection.
The EL731 can be applied to a
grounded PV system to detect ground fault-
current levels that are well below
the fuse rating, thereby lowering the
levels of ground-fault detection and
creating a much safer system. An EL731
is installed in the inverter, with the
current sensor installed to monitor the
negative-ground path through the fuse.
In this location, an EL731 would detect a
ground fault on the ungrounded bus
anywhere on the system. Since the
EL731 is much more sensitive than the
grounding fuse, it is also much more
likely to detect a ground fault on the
grounded bus as well. However, the
amount of ground-fault current that
flows during the fault will depend on
the resistance of 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 to
alert maintenance personnel of the
problem. The EL731 has LED trip
indication on its faceplate, and optional
network communications can be used
to remotely send notification of a
ground fault. Ground-fault
coordination is achieved by detecting a
ground fault, removing or isolating the
minimum amount of equipment
required to clear the fault, and allowing
the rest of the system to safely remain
energized. In addition, ground fault
coordination greatly simplifies location
of the fault for the maintenance team.
Proper ground-fault coordination
uses time delays; relays closest to the
system grounding point (inverter) are
set to trip slowest, and relays further
from the system grounding point are set
to trip faster. If the first relay trips (the
EL731 in the string combiner in the
picture above) and removes the fault
before the time delay of the second relay
(the EL731 in the inverter) expires, the
second relay will not trip and the rest of
the system will continue to operate.
The maintenance team would
immediately be alerted to the presence
of a ground fault and would know
which array of strings it is on by seeing
which relay is tripped.
To achieve ground-fault coordination
over the entire system, one EL731 and
one contactor or breaker is installed in
each combiner box. Each EL731 only
detects a ground fault in the array of
strings connected to its combiner box; it
would not detect a ground fault in any
other array of strings. When a ground
fault occurs in an array, the
corresponding EL731 will trip; the
relay's output contacts will then trip a
switch, breaker, or contactor to remove
the faulted array from the system.
An EL731 installed in the inverter is
backup protection and also 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
first.
A current-sensing relay, such as the
EL731, requires the system to remain
grounded long enough to detect the
fault and to allow any programmed triptime
delays to expire. Since the
programmable trip-time delay in the
EL731 is 0 to 2 seconds, a ground-fault would remain on the system for a
maximum of 2 seconds. If the fuse opens
before the relay trips, the ground-fault
current goes to zero, and the relay, no
longer detecting ground-fault current,
will not trip. To guarantee ground-fault
coordination, a 5A current-limiting
resistor, R, is connected in parallel with
the grounding fuse.
When the fuse is closed, R will be
shorted out and has no effect on the
system; current-sensing ground-fault
relays will operate normally. If a high current
ground fault occurs and causes
the fuse to open, the current-limiting
resistor R will allow a controlled
amount of ground-fault current for the
relays to detect and isolate the fault.
Once the faulted array is isolated , the
current through the resistor R will
return to zero.
UNGROUNDED SYSTEM
An ungrounded system is defined as
having no intentional connection to
ground. An ungrounded system
presents a problem for ground-fault
detection. Since the bus has no
intentional connection to ground, a
ground fault will not close a loop and
ground-fault current will not
flow.Current-sensing ground-fault
relays cannot be used. It is worth noting
that when the first ground fault occurs
on an ungrounded system, the system
now becomes a grounded system
through the fault until the fault is
repaired. This is not desirable as a
subsequent ground fault occurring on
the unfaulted bus will cause a busground-
bus short circuit, resulting in
short-circuit currents, with possible arcflash,
fire, and shock hazards.
On an ungrounded system, insulation
monitors are typically used to measure
bus-to-ground resistance. With no
ground fault, the monitor will measure
a very high value. When a ground-fault
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
difficulties; the first is that the
capacitance of the system to ground
presents a path to ground to the
insulation monitor, and if large enough,
will cause a nuisance trip. Since
capacitance is a function of 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 fault
anywhere on the system.
Troubleshooting and fault location are
difficult and time consuming.
RESISTANCE-GROUNDED
SYSTEM
The 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 safest
and most stable distribution system is
high-resistance grounded. A high resistance-
grounded system uses a
neutral-grounding resistor to connect
the neutral point of a wye (star)
transformer secondary to ground. The
benefits of a high-resistance-grounded
system are limiting ground-fault
current to a low level, elimination of
transient over voltage, elimination of
arc flash hazards caused by a ground
fault, and the ability to use selective
coordination for ground-fault
protection. Many of these benefits can
be achieved on a DC system by
grounding with a resistor network. In
addition, a ground fault on either bus
can be reliably detected on a resistance grounded
system.
To implement high-resistance
grounding on a DC system, a "neutral"
or zero-voltage point must be first
established. This can be achieved
through the use of matched resistors, 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 micro processor based
ground-fault monitor for dc
systems, designed to monitor a
resistance-grounded system.
The trip level of the ground-fault
circuit is selectable from 1 to 20mA,
and trip time is selectable from 0.05
to 2.5s.
When used with the PGR-2601, the
resistor network is designed to limit the
ground-fault current to 25mA, and to
allow a nominal unfaulted current of
12.5mA. The total required resistance
between the positive and negative bus is
calculated using Ohm's law; R
Total=V/I. For example, on a 1,000-Vdc
bus, the total required resistance is R
Total = V/I = 1,000/0.0125 = 80k?.
When there is no ground fault on the
system, the voltage at the centre point
of this resistor network, S, is 0 V, and no
current flows through the S-to-relay toground
conductor. When a ground fault
occurs on either bus, the voltage across
one resistor rises to full bus voltage,
causing 25mA of current to flow
through it, the S-to-ground connection,
and the PGR-2601. The PGR-2601
detects this current and trips. To achieve
proper ground-fault detection and
coordination for both buses, a resistor
network can be used along with an
EL731 and breaker or contactor at each
combiner box.
CONCLUSION
A grounded system is superior to an
ungrounded system as it allows the use
of current-sensing ground-fault relays
to quickly and accurately detect low
levels of ground-fault current. A
system with one bus grounded through
a fuse and current-limiting resistor
allows ground-fault protection for the
ungrounded bus; however, groundfault
detection and protection for the
grounded bus is not guaranteed. A
system that is resistance grounded
through a resistor network is the best
solution, as it allows ground-fault
detection and protection for both
buses. Relays and disconnect
mechanisms are installed at each
combiner box to detect a ground fault
and remove the minimum amount of
equipment to clear the fault. Not only
is this system the safest, it is the most
convenient to maintain and will reduce
system down time.
(Tyler Klassen, P.Eng, is Sales
Engineering Manager, Littelfuse Inc)
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