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Power topic #5590 | Technical information from Cummins Power Generation
Reliability considerations in
simple paralleling applications
■ White Paper
By Rich Scroggins, Technical Specialist, Sales Application Engineering
Reliability in power generation
systems, defined as the probability
that power will be available at any
point in time, is the primary reason
standby generator sets are purchased.
Using paralleled redundant generator
sets is one method commonly used
to enhance system reliability.
Redundancy traditionally has been a
requirement only in critical applications
such as data centers and hospitals
where an extended loss of power
could result in loss of life or a substantial financial loss, as these were
the only scenarios where the cost of a
redundant generator and the associated paralleling switchgear could be
justified. The availability of lower cost
power transfer devices and paralleling
control systems have in recent years
made redundant paralleled generators
an attractive option in less critical
standby power applications.
Single Generators vs.
Redundant Paralleled
Generators
The decision on whether to use a single generator
set or multiple paralleled generator sets will typically
be based on reliability and cost. When a decision
is made to use paralleled generator sets there are
many considerations that need to be addressed to
ensure a reliable system.
Reliability and Redundancy
The purpose of redundancy is to eliminate a single
point of failure from a system. It is well documented
that having redundant systems will make the overall
system more reliable however this is always based
on the assumptions that single points of failure
are truly eliminated and not just moved to another
part of the system and that the controls enabling
redundancy don’t introduce new failure modes
which compromise reliability. Paralleled generator
sets that rely on a single master control for signals to
start and to close to a paralleled bus actually replace
one failure point with two as the master control and
the communication link between the master and the
generator sets each represent single points of failure.
A standby generator set from a reputable
manufacturer that has been maintained properly
and tested periodically according to manufacturer’s
recommendations is a very reliable solution. Adding a
redundant generator with the inherent complexity of a
paralleling system isn’t necessarily going to make the
system more reliable. Investing in a reliable generator
set and a robust maintenance program so the
generator doesn’t fail is often a better investment than
installing a more complex system to compensate for a
failed generator set.
Total System Cost
There are some instances where the cost of two small
generator sets will be less than the cost of one larger
generator set. The total installed cost of the system is
often overlooked in basic standby applications. There
are many factors which need to be evaluated beyond
the cost of the generator sets.
A larger generator set may require
additional structural support as the weight of
the generator set will be concentrated on one
spot, however smaller generator sets may require
pouring multiple concrete slabs.
■ Foundation:
requirements: Multiple generator sets
and their associated switchgear will take up more
space than a single larger generator set although
the smaller generator sets offer greater flexibility as
individual generator sets can be maneuvered into
smaller spaces than a larger set.
■ Space
Smaller generator sets enable the use
of smaller cable and easier termination however
paralleled generator sets will require additional
cable runs which will be labor intensive, particularly
if cable is run underground.
■ Cabling:
costs: Startup and testing costs
of paralleled generator sets are substantially higher
than those costs for a single generator set.
■ Commissioning
costs: Replacement parts for
smaller generators will be less expensive than
replacement parts for a larger generator however
that difference is more than offset by the labor
costs of maintaining two generators and switchgear
rather than one single generator set.
■ Maintenance
investment: When the power demanded
at a facility is expected to increase in the future
initial capital investment can be minimized in
some cases by installing a smaller generator with
the intent of adding paralleled generators in the
future as demand increases rather than installing
a single larger generator that is oversized for the
load. This will need to be balanced against the
future investment required to add generators and
switchgear and other required facility modifications.
02 | Power Topic #5590
■ Capital
Paralleling Systems
When a decision has been made to parallel generator
sets there are several concerns that need to be
addressed to ensure that the system is as reliable as
possible.
Control System
A robust control system is critical to having a reliable
paralleling system. A control system needs to minimize
single points of failure and have fault tolerance
measures built in. Key factors in a paralleling control
include the following:
Eliminate single points of failure where possible
The most effective way to eliminate single points
of failure in a control is to use distributed logic and
control rather than centralized control. Critical control
functions such as generator starting, bus voltage
sensing, synchronizing and closing to the bus should
be executed by individual generator set controls rather
than a master control. This way the system will have
redundancy in critical control functions in addition to
having redundant generators and the single point of
failure is eliminated.
starting: In a simple standby isolated
bus paralleling application the start signal is sent
directly from the transfer switches that sense the
utility failure to the generator sets. Sending the
signal through a master control adds no value and
introduces an unnecessary failure mode.
■ Generator
bus voltage sensing: For reliable
paralleling each generator must sense the bus
voltage independently rather than rely on a signal
from a separate control
■ Paralleling
to a dead bus: Generator controls should
determine when to close their paralleling breaker
to the bus. To provide the fastest and most reliable
service to a dead bus generators must arbitrate
between each other so that only one generator
closes to the bus. Waiting for a permissive signal
from a master slows the system down and adds an
unnecessary failure mode
■ Closing
■ Synchronizing
and closing to a live bus:
Generator sets synchronize reliably and quickly
when there is no other control in the loop. External
controls adjusting bias lines or otherwise interfering
with the synchronizing algorithm introduce
unnecessary complexity into the system.
SIMPLE LOAD ADD SCHEME WITH TWO LEVELS
Transfer
Inhibit
Paralleling
Breaker
Auxiliary
Contact
Generator Set 1
Paralleling
Breaker
Auxiliary
Contact
Non-Emergency ATS
Generator Set 2
Figure 1: Implementing a load add function.
Load add and load shed
Load add and load shed schemes are used to make
sure that there is always sufficient capacity to serve
the most critical loads. Two levels of load add (one
level for emergency loads and one level for all other
loads) and one level for load shed (emergency loads
are never shed) is sufficient for most simple, isolated
bus paralleling applications. This can be implemented
without the use of a master control. A master control
may be required for additional levels of load add/shed.
Although a master control can present a single point of
failure the system can be designed so that failure of the
master will not impact the most critical loads.
A load add scheme is required in a paralleling system
when a single generator is not large enough to carry all
of the loads in the system. A simple load add scheme
with two levels can be implemented using the inhibit
function of the non-emergency load transfer switches
and the aux contacts of the genset paralleling breakers
(Figure 1). Emergency load transfer switches should
not be inhibited and should close to the bus as soon
as it is live. Non-emergency transfer switches can be
inhibited until all of the generator sets come on line.
A load shed scheme is required so that when
generators are overloaded the non-critical loads can
be taken off line so that there will be sufficient capacity
to serve the critical loads. Most paralleling generator
set controls have a load shed or load dump output
which can be connected to the load shed input of the
transfer switches that serve the non-emergency loads
(Figure 2). This will take the non-emergency loads off
line in the event that the generator sets are overloaded.
Note that to properly shed load transfer switches must
be three position switches with center-off positions.
Isochronous vs. droop load sharing
Most load sharing systems today are isochronous,
meaning that the voltage and frequency are held constant
however there are still controls being produced that use
droop load sharing, which allow voltage and frequency
to vary with load. Droop load sharing controls were once
popular because they allowed generator sets to parallel
with each other without communicating with each other.
Due to the variation of frequency and voltage with a
droop paralleling system the quality of power provided
to the load typically is not very good and may not be
suitable for some electrical equipment. Isochronous
load sharing is the appropriate technology to use.
IMPLEMENTING A LOAD SHED FUNCTION
Load
Dump
Contact
Generator Set 1
Load
Dump
Contact
Non-Emergency ATS
Generator Set 2
Figure 2: Most paralleling generator set controls have a load shed or load dump output which can be connected to the load shed input of the transfer
switches that serve the non-emergency loads.
03 | Power Topic #5590
Load
Shed
Random access paralleling vs. exciter paralleling
Random access paralleling refers to a system in which
the first generator at rated speed and voltage closes to
the dead bus and then all the other generators actively
synchronize and close to the bus. Random access
paralleling is the most reliable paralleling method and
is most commonly used in critical applications. A
less expensive paralleling method known as exciter
paralleling is used in some paralleling applications.
In an exciter paralleling system all of the generators
start with their paralleling breakers closed and their
excitation circuits de-energized. When the generators
start they are connected to the bus but produce no
voltage. When all generators reach starter disconnect
speed their excitation circuits are energized and the
generator bus voltage ramps up with the generators
forcing each other into sync. Because exciter paralleling
systems will not work until all generators either reach
disconnect speed or are locked out they are not used
in critical applications. Random access paralleling with
active synchronization should always be used when
paralleling gensets in critical applications.
Fault tolerance – manual mode
A key consideration in assessing the reliability of a
system is the ability of the system to operate when
certain components have failed. Having a pre-defined
sequence of operations for a user to follow in a worst
case scenario to manually provide power to loads
is often a requirement in critical applications. A user
should be able to manually start generators, initiate
synchronization and close paralleling breakers. Manual
operation does not mean that the generator control
is not operating. It means only that functions are
user initiated rather than system initiated. All system
protection functions will still be active. The control will
not allow a paralleling breaker to close if the generator
and the paralleling bus are not in phase with each other.
Product Standards
04 | Power Topic #5590
Consulting engineers in the US consistently specify
that paralleling switchgear must be rated to UL 891
or UL 1558. These standards ensure that industry
safety guidelines are being followed and that the power
transfer and protective devices and bussing have all
been appropriately evaluated as a system for safety
under fault conditions.
Power transfer devices must also be rated
appropriately for the paralleling application. UL 891
and UL 1558 specify circuit breakers listed to UL 489
(in the case of UL 891) or UL 1066 (in the case of UL
1558) for overcurrent protection and the breakers
are typically used for power transfer as well as for
protection. Draw out breakers are specified in critical
applications such as hospitals and data centers,
allowing operators to isolate faults and facilitate
inspection and testing of breakers and easily replace
breakers if necessary.
Contactors that are not listed to a standard for
paralleling equipment may not be suitable for a
paralleling application as they may not have been
evaluated for safety when subjected to the higher
levels of fault currents present in a system with
paralleled sources.
Contactors that are components in UL 1008 listed
transfer switches may not be suitable as a power
transfer device in a paralleling application. UL 1008
recognizes and lists the entire switch mechanism,
not the individual contactor. The individual contactor
has not been type-tested according to the UL
requirements and does not carry the UL listing
separate from the transfer switch mechanism. In
addition, UL 1008 lists switches that transfer loads
between sources but does not test or recognize
devices that parallel between two live sources.
Because of these two factors it is not appropriate to
apply the UL 1008 standard to contactors that are
used to parallel between two live sources.
Installation Considerations
Installing and commissioning paralleled generator
sets is not a simple process. A qualified manufacturer
will have experience with protective relaying,
system grounding, and other paralleling issues
beyond the generator set functionality. Working
with a manufacturer who has substantial paralleling
experience over a wide range of applications and
will assume responsibility for a correct installation is
a key to a successful project even in the most basic
paralleling application. There are several considerations
that an experienced installer will address.
Selective coordination
The National Electrical Code requires selective
coordination for emergency and legally required loads.
Any downstream breakers must coordinate with
upstream overcurrent protection such as paralleling
breakers or a genset mounted breaker. Coordinating
with a generator set mounted Molded Case Circuit
Breaker (MCCB) with an instantaneous trip will be
very difficult and will require in most cases that the
downstream breakers are supplied from the same
breaker manufacturer as the genset mounted MCCB.
It is much easier to coordinate with a power breaker
as is most often used in paralleling switchgear as
they are typically equipped with programmable trip
unit specifically for the purpose of coordination.
When the generator control includes integral, UL
listed overcurrent protection, coordination between
the genset and the paralleling breaker is simplified
because the overcurrent trip curve is optimized to
allow the maximum permissible time delay while still
protecting the alternator.
Separation of circuits
The National Electrical Code requires that Emergency,
Legally Required and Optional loads are separated
from each other. With paralleled generator sets
that means that the Emergency, Legally Required
and Optional loads must be fed from the generator
paralleling bus by separate breakers in separate
compartments or sections in the switchgear line up.
Isolation of generators from the paralleling bus
To enable maximum reliability and safety there must
be means to individually disconnect each generator
from the paralleling bus located at the paralleling
switchboard. Without this disconnecting means,
typically an incoming breaker, a fault on one generator
can make all generators inoperable and all generators
will have to be locked out to do maintenance work
on any generator in compliance with NFPA 70E Lock
out tag out requirements (Figure 3). Without this
disconnecting means much of the value of having a
redundant generator will be lost.
DISTRIBUTION PANEL – NO INCOMING BREAKER
G
The contractor and consulting engineer must have
a clear definition of what is included in the scope of
supply for each part of the system. Dividing scope
of supply won’t be limited to equipment but will also
include assignment of responsibility for meeting code
requirements like selective coordination and separation
of circuits, system testing and start up and ongoing
maintenance and service. Having the same entity
responsible for supply, commissioning and maintenance
helps promote an efficient maintenance program.
Service and Support
One of the first questions that should be asked when
choosing a supplier of a paralleling system is how will
the system be supported in the future? Paralleling
systems include engines, alternators, controls,
switchgear and transfer switches and properly
supporting all of this equipment requires a diverse
skill set. Some suppliers only have experience with
one component in a system and will need to involve
other companies to troubleshoot problems that arise.
Working with a supplier that has a proven track record
of designing, installing and maintaining complete
paralleling systems is the best way to ensure reliable
operation over the life of the system. Questions to ask
include
■ Have
the service technicians been trained and
certified by the manufacturer on all components
of the paralleling system? Claims that an engine
dealer technician can service a paralleling system
should be viewed with skepticism.
Distribution Panel
Figure 3:
Ten second start requirement
The National Electrical Code requires that emergency
loads are served within ten seconds of a utility failure.
As ten seconds isn’t enough time to start, synchronize
and close multiple generator sets this requirement
means that each of the generator sets in the system
must be large enough to carry all of the emergency
loads on its own. For example, a system of three
paralleled 600 kW generator sets will not meet the
NEC requirement if the emergency load exceeds
600 kW. To meet this requirement the sequence of
operations should not require any interaction with
a master controller. All control functions should be
carried out independently by the generator and the
transfer switch controls with no communication
required other than a genset start command.
Supplier of distribution switchboard
Sourcing the generator and paralleling controls
separately from the switchboard complicates a project.
■ Have
the service technicians been certified with
the make and model of engines being used in
the application? Suppliers that use engines from
different manufacturers may require different
service organizations to support systems within the
same geographical region.
■ Does
the service organization offer comprehensive
maintenance programs for the entire system?
■ Does
the service organization have a demonstrated
history of supporting paralleling systems in many
different types of applications?
■ What
is the availability of replacement parts? Power
transfer devices will often need to be replaced after
faults. Replacing a proprietary contactor rather than
a paralleling circuit breaker can result in a system
being down for significant period of time. Replacing
a proprietary component with a standard one may
not be acceptable if the new device isn’t listed for
use with the existing overcurrent protection.
■ If
a control needs to be replaced is custom
programming required for the replacement control
and who is authorized to do the programming?
■ What
is the time frame for replacing the control?
05 | Power Topic #5590
G
About the author
Rich Scroggins is a Technical Specialist in the
Application Engineering group at Cummins
Power Generation. Rich has been with Cummins
for 18 years in a variety of engineering and
product management roles. Rich has led product
development and application work with transfer
switches, switchgear controls and networking and remote
monitoring products and has developed and conducted
seminars and sales and service training internationally on
several products. Rich received his bachelors degree in
electrical engineering from the University of Minnesota
and an MBA from the University of St. Thomas.
Scalability
Conclusions
Paralleling systems are frequently expanded after
they are put into service to accommodate increasing
power demands. The ability to add a generator set
and the associated switchgear in the future should
always be considered. The system should have the
flexibility to allow generator sets from a different
manufacturer to be added in the future. Being locked
in to a certain manufacturer limits flexibility for future
expansions. Additional question to ask concerning
expansion include:
The decision on whether to provide backup power
with a single generator set or with redundant paralleled
generator sets will be based on reliability and cost. The
key question is does the redundancy coupled with the
added complexity of a paralleling system increase the
system reliability enough to justify the additional cost?
When a decision has been made to parallel generator
sets there are several considerations that need to be
addressed to maximize the reliability of the system.
■ The
the generators properly isolated so that
new generators can be added without taking
the facility off line?
■ What
is involved in modifying the control for
expansion?
■ If
a different manufacturer’s generators are used
for the expansion what will be required for the
generators to parallel properly?
control system should be designed with critical
functions distributed to the individual generator
controls to minimize single points of failure
■ The
controls should have fault tolerance provisions
such as load shed and manual operating modes.
■ The
installation must meet code requirements for
coordination, separation of circuits and ten second
service to emergency loads and must allow proper
isolation of the generator sets.
■ Does
■ The
■ How
For additional information about onsite power systems
or other energy solutions, visit power.cummins.com.
the manufacturer have experience
implementing field expansions, including
expansions that include generators from other
manufacturers.
can the system be modified to support
utility paralleling if that is required in the future?
system must be supported by an organization
with a proven track record for servicing complete
paralleling systems.
power.cummins.com
©2013 Cummins Power Generation Inc.
All rights reserved. Cummins Power Generation
and Cummins are registered trademarks of
Cummins Inc. “Our energy working for you.”
is a trademark of Cummins Power Generation.
GLPT-5590-EN (09/13)
06 | Power Topic #5590
■ Are