Part 1:
Existing Systems
Preventing ground faults and alerting when
potential problems arise.
INTRODUCTION
The demand for design and construction services for commercial solar
photovoltaic (PV) systems has increased dramatically over the last several
years. This increased demand has resulted in many systems being
designed and built by companies with relatively little experience. In addition,
increased competition has resulted in some vendors sacrificing quality and
safety to reduce system design and construction cost. Even though fires
are not common at commercial solar PV systems, these deficiencies could
make fires more frequent. For that reason, it is even more important to
make sure that fire safety is given the proper consideration during design
and construction of solar PV systems.
Faults (undesired current flow paths) can occur in any electrical system,
including PV systems. A fault is a malfunction in the insulation surrounding
a conductor. Causes of faults include, but are not limited to, construction
deficiencies, corrosion, and physical damage. Faults will have some
resistance to current flow. A major fault will develop a small amount of
resistance in a very short time, but faults usually have a high initial
resistance compared to the normal circuit. Faults can also occur across air
by generating an arc to complete the circuit. A direct current (DC) arc may
have a relatively high resistance.
“Even though fires are not
common at commercial
solar PV systems, these
deficiencies could make
fires more frequent”
Since solar modules will generate power any time a sufficient light source is
available, the DC section of a solar system may remain energized even
after shutting down the inverters and opening the disconnects for the
alternating current (AC) and DC circuits. A code compliant solar PV system
does not require an automatic feature to isolate the DC wiring at the
combiner level. Two or more faults in the DC circuits can cause undesired
current flow paths in the system, which can cause a fire, either by
overheating at the fault location(s) or by overloading circuits with excessive
current flow.
The DC circuits of a large commercial solar system are equipped with a
ground fault detection circuit in the inverter. This circuit typically consists of
a low-current fuse that connects the DC equipment ground conductors to
the grounded system conductors. A fault to ground of a DC conductor will
allow current to flow through the equipment grounding conductors and
return to the system through the ground fault detection circuit. Sufficient
fault current flow will blow the fuse. If the fuse blows, the inverter will
automatically shut down. Unfortunately, leakage currents generated in the
© 2013 PowerSecure | 1
system cause a detectable current flow in the ground fault detection circuit
during normal system operation (Nelson 13-15) (Dhere, Pethe and Kaul
Slide 4). This current flow can vary depending on the size of the system,
module technology, the presence of moisture, or changes in humidity or
temperature. Because of this variation, the level of current required to
indicate a ground fault is set relatively high (typically 2 to 5 amps on large
central inverter systems).
“Unfortunately, leakage
currents generated in the
system cause a
detectable current flow in
the ground fault detection
circuit during normal
system operation”
This creates a problem for large PV systems in the United States. The
current version of NFPA 70 (the National Electrical Code) allows the DC
circuits of a system to have one conductor grounded (NFPA 70-603). In this
case, a substantial fault on the grounded conductor could easily go
undetected because a fault on the grounded conductor will create a circuit
parallel to the normal current flow path at the same voltage drop. The fault
path will typically have significantly more resistance than the normal current
flow path, and since the voltage drop is the same for both paths, the
majority of the system current will still follow the normal path. The ground
fault fuse will not blow, because the current that flows through the fault and
the ground fault fuse will be a fraction of the total current and not enough to
blow the fuse.
For example, suppose a system has a ground fault fuse rated at 5 amps. A
grounded DC output circuit conductor for this system has 300 amps of
current flow and a resistance of 0.2 ohms. In two parallel DC circuits,
Ohm’s law tells us that the ratio of the currents in the two circuits is
inversely proportional to the ratio of the resistances. Current flow through
the fault must be at least 5 amps to blow the fuse. This means that the
resistance at the fault must be less than 12 ohms in order for there to be
sufficient current to blow the ground fault fuse.
Even if a ground fault is detected and the inverter is shut down, current can
still flow in the DC circuits. Since the DC circuits are not required to be
isolated except at the inverter DC bus outputs, all of the DC output and
source circuits serving a single inverter are connected through the input
busses and remain connected even after the inverter shuts down and the
main DC disconnect is opened. This means that a combination of two or
more faults to ground can allow current flow from the entire array though
the faults. Fuses in the combiners may not blow in such a situation,
because the current flowing through the combiners could be normal in
magnitude; depending on the location of the fault, the current may actually
flow backwards in parts of the circuit.
“In light of this
phenomenon, the purpose
of this report is to present
solutions to improve the
reliability and safety of
existing commercial solar
PV systems and the
buildings on which they
are mounted.”
In light of this phenomenon, the purpose of this report is to present
solutions to improve the reliability and safety of existing commercial solar
PV systems and the buildings on which they are mounted. It will examine
some conditions that cause undesired DC current flow in solar PV systems
by evaluating fault incidents at several large commercial solar PV systems.
Some of these faults resulted in fires. Suggested actions and modifications
to improve existing systems are presented in detail. Changes to design and
construction of future systems will be discussed in Part 2 of this report. The
intent is for solar PV system designers, installers, operators, and owners to
use this information to improve the safety of existing and future commercial
solar PV systems.
© 2013 PowerSecure | 2
Note that small PV systems, both residential and commercial, generally do
not have large current leakage to ground. The ground fault detection
circuits in these systems can detect ground faults of 1 amp or less. This
means that small ground faults, even on the grounded conductors, can be
detected easily in a small PV system. Therefore, the systems evaluated in
this report are limited to large central inverter designs. However, many of
the recommendations in this report are applicable to these smaller systems.
While the fire risk is lower for smaller systems, it is still strongly suggested
that the applicable recommendations be considered for implementation.
© 2013 PowerSecure | 3
Examples
Several recent events at commercial solar PV systems in the US are
relevant to this report. These events are summarized here. For clarity, each
system is referred to by a generic name. Whenever a particular system is
referenced, its generic name is underlined.
Existing Solar PV System, Commercial Building Roof
Bakersfield, CA, April, 2009
BAKERSFIELD
A second ground fault due
to an improperly installed
conduit expansion joint
triggered the fire
A fire started at a 383 kW PV array on a large commercial building in
Bakersfield, CA. While damage to the PV system was extensive, damage to
the building was minimal and confined to the roof. No injuries occurred as a
result of this fire. The cause of this fire was determined to be two separate
faults to ground in the DC wiring. A ground fault on the grounded conductor
had existed on the system for some time but remained undetected because
the amount of fault current was too low to clear the ground fault fuse. On
the day of the fire, a second ground fault occurred due to an improperly
specified and installed conduit expansion joint. These two faults allowed the
DC current generated by the entire array to pass through the grounded
conductor for a single string at the location of the first fault. This current
overloaded the string conductor and started the fire. A second smaller fire
also started at the location of the second fault due to heating by the arc at
the fault location (Brooks) (Jackson).
The post fire investigation identified several important issues:
− Even a low resistance fault on the grounded conductor may not be
sensed by the ground fault detection system. This is because the normal
current flow path has extremely low resistance so that very little current will
flow through the fault and the ground fault detection circuit.
− Even though the ground fault fuse blew when the second fault occurred
and the inverter properly shut down as a result of the ground fault, the fire
was not prevented because these automatic actions did not interrupt the
DC current flowing to the faults.
− The raceway for the system conductors on the roof was Electrical
Metallic Tubing (EMT). Some of the compression fittings for the EMT were
not properly tightened. Investigators suspected that the tubing was not
inserted correctly into the fittings.
− Expansion joints in the conduit runs were insufficient to prevent thermal
movement from damaging conductors and conduit.
− There was no easy way to electrically isolate the module strings from
one another. Until an electrician was called to the site to remove the fuses
from the combiners, a portion of the DC wiring remained energized.
© 2013 PowerSecure | 4
Existing Solar PV System,
Industrial Building Roof
Carolinas, April, 2011
SITE A
A fire started at a ~1 MW PV array on a large industrial building in the
Carolinas. Damage to the PV system was extensive, but damage to the
building itself was minimal. No injuries occurred as a result of this fire. The
system owner retained PSS and engineer Bill Brooks to investigate this fire.
Once again, investigators determined the cause of this fire to be two faults
to ground in the DC wiring. A difference between this event and
the Bakersfield event is that both faults at Site A occurred on DC output
conductors that were rated for higher current flow than the string
conductors. The fire appears to have started as a result of heating caused
by arcing at the fault locations. As with the Bakersfield incident, there were
actually two fires at Site A, one at each of the two fault locations (Brooks,
Proprietary Fire Investigation Report).
The important issues discovered during this incident investigation were:
− Again, the inverter served by the faulted array properly detected a
ground fault and automatically shut down as a result. However, the fire still
occurred because the inverter shut down did not interrupt the DC current
flow to the faults.
Insulation damage went
undetected by the ground
fault protection fuse in the
inverter
− Investigators discovered damage to the insulation of the DC conductors
at several locations on the roof of the building. Some of the insulation
damage may have occurred as a result of improper installation of the cable
in the conduit, while some of the damage may have occurred because of
insufficient allowances for thermal movement of the conductors and
conduits causing chafing of the conductor insulation at bushings.
− The raceway system was not electrically continuous. There were many
conduit couplings without bushings and there were several non-conducting
junction boxes used without equipment grounding jumpers to electrically
connect the conduit in the boxes. This caused the faults to travel long
distances along the metallic raceway to reach a grounding point. Since
there could have been additional arcing and heating where current was
passing through high resistance sections of the raceway system, this
situation could have led to additional fires at other locations on the roof.
− As in the Bakersfield incident, expansion joints in the conduit runs were
insufficient to prevent thermal movement from damaging conductors and
conduit. In addition, poor wire management may have worsened the
damage to the conductors.
− Again, it was difficult to isolate the strings from one another. In this
case, the fuses in the combiners were not removed under load. Instead,
the fuses were removed after sunset, when the generation from the panels
stopped.
© 2013 PowerSecure | 5
New Solar PV System, Industrial Building Roof
Mid-Atlantic, May, 2011
SITE B
“The system had operated
while this fault existed, but
the ground fault detection
system did not sense this
fault.”
As part of the commissioning of a new ~700 kW commercial solar PV
system, the PSS commissioning team conducted insulation tests on all of
the DC conductors. This testing was accomplished by lifting the conductors
at the main DC busses at the inverters, and then testing each circuit using a
mega ohm tester. A fault was discovered on a grounded DC output circuit
conductor. The fault was insulation damage that was apparently caused by
improper installation of the cable into the conduit. The system had operated
while this fault existed, but the ground fault detection system did not sense
this fault. The total current flowing through the ground fault detection circuit
was 2.3 amps, which was insufficient to blow the fuse. This fault was
repaired and the system was restored to service.
The important issues from this incident are these:
− As at Bakersfield, a fault on the grounded conductor may not be sensed
by the ground fault detection system, even on a large output circuit.
− Preoperational testing of large solar systems should include insulation
testing of all conductors.
− Cables should be inspected for insulation damage immediately after
installation. Cables with damaged insulation should be replaced.
− Timely commissioning of the system uncovered a problem that could
have led to a fire. In an existing system, preventive maintenance can detect
a problem like this before it results in system damage.
© 2013 PowerSecure | 6
Existing Solar PV System,
Commercial Building Roof
North Carolina, May, 2011
SITE C
In response to the fire at Site A, PowerSecure Solar personnel were asked
by the system owner to investigate Site C, which is a ~500 kW existing
solar PV system located on the roof of a commercial building in North
Carolina. Assisting with this investigation were Bill Brooks, PE, of Brooks
Engineering, Vacaville, CA; an engineer representing an instrument
manufacturer; and engineers working directly for the system owner.
The first activity was insulation testing of all of the DC conductors for the
system. PSS shut down the inverter to perform the testing. This system is
equipped with disconnecting combiners, so all of the DC circuits were
opened at the disconnects in the combiners. Then, the DC conductors at
the main DC busses in the inverter were lifted. Each of the circuits feeding
the main DC busses was then tested for faults using a mega ohm tester. No
faults were detected on the system. PSS then restored the system so that
an additional test could be performed.
No faults were found at Site C,
and it was subsequently
recommissioned with a
differential current monitor
The next activity was testing of a differential current monitor (also called a
residual current monitor or RCM). This device uses a current transformer
(CT) to check for a difference in current between the grounded and
ungrounded DC conductors. The RCM is quite sensitive, and can detect
differences in current as small as 10 mA. PSS installed the CT at the DC
output circuits near the DC input busses in the inverter. Then, simulated
ground faults were placed on the system using jumpers with varying size
resistors. The RCM was able to detect these simulated faults based on the
difference in current between the grounded and ungrounded conductors.
After the successful test of the RCM, PSS permanently installed this device
in the system. The monitor has 12 channels supporting 12 CTs. These CTs
were installed so that they are monitoring all of the DC output circuits from
the array. The CTs for this particular system are able to serve multiple sets
of conductors, so a 12 channel RCM can provide ground fault detection
capability for at least 24 DC output circuits. The RCM was wired to provide
an e-stop signal to the inverter. An e-stop will notify the owner by sending a
signal to the owner’s monitoring station.
Because of the addition of this RCM to the system, the system at Site C is
now capable of detecting very small ground faults and automatically
responding to a detected fault by shutting down the inverter and notifying
the owner of the problem. This notification will allow the system owner to
respond and repair the fault promptly, but the system owner must still take
action to repair the system before a second fault occurs.
© 2013 PowerSecure | 7
Solutions
“These programs will identify
issues that may lead to system
faults…”
PowerSecure Solar proposes several solutions to increase the safety of
both new and existing systems. For new systems, appropriate solutions
should be implemented by the system designer in the design phase. For
existing systems, we propose solutions that will provide greater system
safety with minor modifications.
NEW SYSTEM SOLUTIONS
There are a number of steps that can be taken in new system design and
construction that will provide greater protection from the types of events
described above. PowerSecure Solar is already implementing many of
these solutions. In part 2 of this report PSS will present these new system
solutions.
EXISTING SYSTEM SOLUTIONS
There are many existing systems in operation today. A significant number
of these may be vulnerable to the types of events described above.
PowerSecure Solar recommends that all system owners engage an
experienced and qualified installer or engineer to evaluate whether the
services outlined below would improve the safety and reliability of the
system. PSS offers these services and is prepared to implement them.
PREVENTIVE MAINTENANCE PROGRAM
By their nature, solar systems expose a large amount of electrical
equipment and components to harsh outdoor conditions. This equipment is
exposed to sunlight, temperature extremes, wind, precipitation, dirt,
animals, and more. Because of this, even the best designed and installed
systems will sometimes sustain damage.
A robust, twice annual preventive maintenance program can prevent minor
system damage from becoming a fire or other significant event. These
programs will identify issues that may lead to system faults, including
problems with conduit, fittings, insulation, expansion joints, and other
components. In addition to visual inspections, preventative maintenance
should include insulation testing of all conductors that are susceptible to
environmental damage.
In addition to finding and repairing damage, preventive maintenance should
also include steps that will improve system performance, including cleaning
and alignment checks of the modules. By using a good preventive
maintenance program from a knowledgeable and skilled provider, system
owners can ensure peak performance, reliability, and safety for the life of
their system.
© 2013 PowerSecure | 8
DIFFERENTIAL CURRENT TESTER WITH
CONTACTOR COMBINERS
“The system will be
arranged so if it detects a
ground fault, the RCM will
immediately stop the
inverter, open the contacts
for all of the combiners
serving that inverter,
provide a signal that can
be used to notify the
owner, and provide a local
alarm.”
After evaluating the background incidents, PowerSecure Solar has reached
the conclusion that the best way to protect a PV system from DC circuit
faults is to retrofit the system so that it takes automatic protective action if a
ground fault occurs. By implementing this modification, the system will be
continuously monitored for any ground fault on either the grounded or
ungrounded conductors. If the system detects a ground fault, it will
automatically isolate the DC circuits, stop the inverter, provide a local
alarm, and notify the owner of the event.
The proposed modification also meets these factors that make it ideal for a
retrofit to an existing system:
−
The power output of the system is unaffected.
−
The outage required for the modification is short.
−
The cost of the modification is relatively small.
−
All of the new components are code compliant and qualified as
required.
The modification will make these changes to the system: A residual current
monitor (RCM) along with current transformers (CTs) will be added to the
system. The function of the monitoring package and details about
installation will be identical to the modification already implemented at Site
C and described in the background above. Along with installation of the
monitoring package, the combiners will be replaced with contactor
combiners. A contactor combiner uses an electrically held relay to close a
contact on the ungrounded output of the combiner. On loss of power to the
solenoid, the relay will open the contact and isolate the combiner from the
rest of the DC system. The system will be arranged so if it detects a ground
fault, the RCM will immediately stop the inverter, open the contacts for all of
the combiners serving that inverter, provide a signal that can be used to
notify the owner, and provide a local alarm.
© 2013 PowerSecure | 9
Conclusion
“It is important to the industry
as a whole to make PV
systems as safe and reliable
as possible. We urge all
owners of existing systems to
implement these two solutions
right away.”
Ground faults on the DC source and output circuits of a large solar PV
system, while relatively rare, can cause significant damage to the system
and to the building on which the system is installed. In addition to this, the
existing ground fault detection systems will not detect some ground faults in
large inverters currently used in the US. While poor design or installation
may cause faults, the environment in which a solar system operates is
harsh and can cause these faults without human error being involved.
Because of these issues, PowerSecure Solar believes it is important for all
owners of large systems to implement two solutions that, together, will
provide a greater level of protection from DC circuit faults. First, a robust,
twice annual preventive maintenance program will monitor for problems and
ensure peak system efficiency. Second, a modest modification to existing
systems will provide reliable detection and automatic action to limit or
prevent damage resulting from a fault. Implementing these two solutions
will improve the safety and reliability of existing commercial PV systems. It
is important to the industry as a whole to make PV systems as safe and
reliable as possible. We urge all owners of existing systems to implement
these two solutions right away.
© 2013 PowerSecure | 10
Bibliography
Brooks, B. P. (2011). Proprietary Fire Investigation Report. Vacaville, CA:
Brooks Engineering.
Brooks, B. P. (2011, February/March). “The Bakersfield Fire.” Solar
Professional, pp. 62-70.
Dhere, N. G., Pethe, S. A., & Kaul, A. (2011). High Voltage Bias Testing of
Specially Designed c-Si PV Modules. Golden, CO: Florida Solar Energy
Center.
Jackson, P. E. (2009). [store name] Roof PV Fire of 4-5-09, [address],
Bakersfield, CA. Bakersfield, CA: City of Bakersfield.
NECA. (2010). NECA/ANSI Standard 1 - Standard Practice of Good
Workmanship in Electrical Construction. Bethesda, MD: National Electrical
Contractors Association.
NECA. (2006). NECA/ANSI Standard 101 - Standard for Installing Steel
Conduit (Rigid, IMC, EMT). Bethesda, MD: National Electrical Contractors
Association.
Nelson, J. (2003). The Physics of Solar Cells. London: Imperial College
Press.
NFPA. (2010). NFPA 70, National Electrical Code, 2011 Edition. Quincy,
MA: National Fire Protection Association.
Steel Tube Institute. (2001). Guidelines for Installing Steel Conduit/Tubing.
Glenview, IL: Steel Tube Institute of North America.
UL. (2010). UL-1741 - Inverters, Converters, Controllers and
Interconnection System Equipment for Use With Distributed Energy
Resources. Camas, WA: Underwriters Laboratories, Inc.
© 2013 PowerSecure | 11
Part 1:
Existing
Systems
FIGURE 1: SOLAR PV
SYSTEM SIMPLIFIED
DIAGRAM
The following illustrations are not representative of any specific solar PV
system. Instead, they are diagrams intended to explain the DC wiring
circuits of a typical commercial solar PV system and to show how the
proposed modifications will improve the fire safety of the system.
Figure 1 is a simplified diagram of a typical central inverter solar PV system.
On the left are the strings, which are groups of modules wired together in
series.
Two or more strings are connected together in parallel at a device called a
combiner or combiner box. In this system, for simplicity, there are two
strings each connected together at two combiners for a total of four strings.
An actual system may have 20 or more combiners with 20 or more strings
connected at each combiner. A set of strings all connected to a single
combiner is commonly called a sub-array.
To the right of the combiners are DC disconnects. Disconnects serve as a
way to remove power to the inverter system for maintenance or an
emergency. The inverter accepts the DC electrical energy from the strings
(via the combiners and disconnects) and converts it to AC electrical energy
that is then used on the electric grid.
© 2013 PowerSecure | 1
FIGURE 2: SOLAR
PV SYSTEM
SIMPLIFIED WIRING
DIAGRAM
Figure 2 shows the same system and some of the internal circuits in the
components. The circuits from the strings are brought together at the
combiners. The outputs of the two combiners are similarly brought together
at the inverters. Even though the system shown has been simplified by
reducing the number of strings and combiners, this diagram is sufficiently
detailed to illustrate the effects of ground faults on the system.
The equipment ground for this system is shown schematically. The
equipment ground is used to ground all of the non-current carrying metal
components in the system. The system shown is a negative grounded
system (the negative conductors are the grounded conductors). The
equipment ground is connected to the grounded conductor through the
ground fault detection circuit inside the inverter. This is the sole connection
between the equipment ground conductors and the grounded DC circuit
conductors.
© 2013 PowerSecure | 2
Examples
Several recent events at commercial solar PV systems in the US are
relevant to this report. These events are summarized here. For clarity, each
system is referred to by a generic name. Whenever a particular system is
referenced, its generic name is underlined.
Existing Solar PV System, Commercial Building Roof
Bakersfield, CA, April, 2009
BAKERSFIELD
FIGURE 3:
UNMODIFIED SOLAR
PV SYSTEM WITH
GAULT ON GROUNDED
OUTPUT CIRCUIT
CONTROLLER
Figure 3 shows a single fault on a grounded (negative) conductor. The
particular conductor shown is a DC output circuit that connects one
combiner with a DC disconnect. Assuming that the resistance of the fault is
in the order of 20 ohms or above, the current that returns to the circuit via
the ground fault fuse will be insufficient to blow the fuse. Therefore, this
fault will go undetected.
© 2013 PowerSecure | 3
FIGURE 4:
UNMODIFIED
SOLAR PV SYSTEM
WITH TWO FAULTS
ON OUTPUT
CIRCUIT
CONDUCTORS
If the system from Figure 3 is allowed to operate with the fault, it is
possible that a second fault could occur, this time to an ungrounded
(positive) conductor. Figure 4 shows a second fault on another DC
output circuit. Even a relatively high resistance fault on the
ungrounded conductor will blow the ground fault fuse, and the inverter
will shut down when it detects that the fuse is blown. If there is
sufficient daylight the strings will still generate electricity even though
the inverter is shut down. Since the energy can no longer travel
through the inverter, it will now all flow through the faults, no matter
what their resistance. Arrows on the above diagram show the flow of
current in the circuits. Some of the current will flow through the busses
in the inverter and then back out (backwards) into the DC system.
This electric energy must be absorbed somewhere, and this will result
in heating at both fault locations and possibly electrical damage and
fires. To stop the flow of electric energy, the circuit must be broken in
some way.
© 2013 PowerSecure | 4
FIGURE 5:
MODIFIED SOLAR
PV SYSTEM WITH
FAULT ON
GROUNDED
OUTPUT CIRCUIT
CONDUCTOR
Figure 5 shows a system that includes a residual current monitor (RCM)
and contactor combiners. An existing system can easily be modified to
include these devices. Note the contacts in each combiner to the right of the
positive busses. The RCM will monitor the current of both the positive and
negative conductors from each combiner. A ground fault will result in some
of the current being diverted through the grounding system, so the RCM will
detect this as a difference between the currents of the two conductors.
When the RCM detects a ground fault in this way, it will send signals to shut
down the inverter, notify the system owner, and open the contact in each of
the combiners. When the contacts in the combiners are open, they will
break the circuit and prevent any flow of energy through this fault.
© 2013 PowerSecure | 5
FIGURE 6: MODIFIED
SOLAR PV SYSTEM
WITH TWO FAULTS
ON OUTPUT
CIRCUIT
CONDUCTORS
Figure 6 shows the system from Figure 5, but with an additional fault.
Recall from Figure 4 that a standard system with these faults will still
generate electrical energy and send it through the faults, possibly
causing a fire. In Figure 6, however, the open contacts in the
combiners have broken the circuit between the strings and the faults,
so no current will flow. This system will, therefore, automatically place
itself in a safe condition until the faults can be repaired.
© 2013 PowerSecure | 6
FIGURE 7: SUBARRAY FROM
MODIFIED SYSTEM
WITH SINGLE
FAULT
Figure 7 shows three strings feeding into a single combiner. Figures 7 and
8 demonstrate how the system will reach a safe condition following two
faults on the DC source circuits serving the same combiner. Figure 7 shows
a single fault on the grounded conductor for one string. Even a low
resistance fault on this type of conductor will go undetected. Only a fraction
of the circuit current will flow through the ground flow path, while the current
flow for a single string (typically in the range of 6 to 8 amps) will not be
enough to trip the ground fault fuse. However, the system shown in Figure 7
is equipped with an RCM, so this fault will be detected, and the RCM will
send signals to shut down the inverter, notify the system owner, and open
the contact in each of the combiners. Since the contact has isolated the
positive conductors from the rest of the circuit, the system will stop
generating electrical energy when the contacts open.
© 2013 PowerSecure | 7
FIGURE 8: SUBARRAY FROM
MODIFIED SYSTEM
WITH DOUBLE
FAULT
Figure 8 shows the same system from Figure 7 with the first fault and
the open contactor in the combiner box, but now there is also a
second fault on one of the ungrounded conductors on another string.
Since this second fault is on a string that feeds the same combiner as
the first faulted string, it has created a circuit through the ground
faults. The open contact in the combiner cannot break this circuit, but
it does prevent energy from the other combiners in the system from
flowing to these faults.
Also, even though the contactor is not protecting this circuit, there is
twice the normal amount of current flowing backwards through the
fuse on the faulted ungrounded conductor. If there is enough sunlight,
the resulting current flow will blow the fuse. Even if there is insufficient
current flow to blow the fuse, the RCM will notify the owner of the
ground fault, giving the system owner some time to respond to the
events before the system or any structures are damaged.
© 2013 PowerSecure | 8
FIGURE 9:
SAFETY
UPGRADES TO
INVERTER
Figure 9 shows a before and after schematic diagram of a large
central station inverter. The “existing” electrical overview shows a
normal negative grounded inverter. The “new” electrical overview
shows the same inverter with the residual current monitor installed. A
CT that measures the current flows to and from a combiner is also
shown. One CT is required to monitor every one or two combiners.
Also shown are the signals that the RCM will send when a ground
fault is detected. The E-stop signal to the inverter will open both the
AC and DC circuit breakers and the main contactor in the inverter.
The other signal will remove power to the solenoids controlling the
combiner contactors, and all of those contactors will open. This makes
the system safe when it detects a ground fault.
© 2013 PowerSecure | 9
GLOSSARY
This glossary provides simple definitions for some of the terms used
in this attachment and in the main article:
ARRAY
A group of strings consisting of modules. Array usually refers to all of
the modules for a system. All of the modules that connect to a single
combiner are sometimes referred to as a sub-array.
CIRCUIT
Conductors arranged to allow current flow. A circuit must form one or
more continuous loops in order to produce current flow. Certain
electrical devices, like a module or an inverter, can form part of a
circuit.
COMBINER
(OR COMBINER
BOX)
A junction box where wires from individual strings are combined into
larger wires to run to the inverter. A combiner typically contains fuses
for the ungrounded conductor for each string.
CONDUCTOR
A component that allows current to pass through it. Electrical cables
are conductors.
CONTACT
An electrical device that can be opened or closed, breaking or
completing a circuit, and thus preventing or allowing current flow.
CURRENT
The flow of electrical energy in a circuit. Current is measured in Amps.
Current is normally measured at a point in a circuit.
DC DISCONNECT
An electrical switch used to interrupt the circuit at the ungrounded
conductor.
FAULT
A malfunction in the insulator protecting a conductor that results in
undesired current flow.
FUSE
An electrical device that will melt or “blow” at a predetermined current
level, thus opening a circuit. Used for circuit protection in the event of
undesired high current.
GROUND
Literally, the earth. Electrical circuits are grounded by being attached
to the earth via buried metal objects like rods, rebar, plumbing pipes,
or building framing. The metal components of an electrical system are
always grounded, so a ground fault may occur between a conductor
and another component of the electrical system (a conduit or junction
box, for example).
GROUNDED
CONDUCTOR
INSULATOR
A conductor that is at the same potential (voltage) as ground. A
properly grounded conductor will always be at zero (0) volts of
potential.
A material that resists current flow, like rubber, plastic, or glass.
Insulation typically surrounds and protects certain conductors like
wires or electrical cables.
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INVERTER
MODULE
The device that accepts the DC power from the modules and converts
it to AC power that electric grid uses.
A single solar panel.
POTENTIAL
The potential for producing current. Potential is measured in Volts.
PV OUTPUT
CIRCUIT
The conductors that connect the combiner to the inverter.
PV SOURCE
CIRCUIT
The conductors that connect the modules together into a string and
connect the string to the combiner.
STRING
A group of modules wired together in series (one module is wired to
the second module; the second is wired to the third, and so on).
UNGROUNDED
CONDUCTOR
VOLTAGE DROP
A conductor that is not directly attached to ground. An ungrounded
conductor usually has a different voltage (either positive or negative)
compared to ground.
The difference in potential between two points in a circuit.
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