Innovus Power
White Paper On
Variable Speed Gensets
M. A. Preston, V1.4, April 2013
Introduction
Present genset design architecture was established in the 1920s, with 99 percent of all today’s
gensets manufactured to operate at one speed to maintain frequency. The limitations of the
technology can be measured principally by efficiency. Efficiency impacts fuel consumption and
carbon emissions, and it impacts longer-term maintenance costs and service life.
However, a historical reduction in the cost of power electronics and an increase in component
reliability have created an opportunity to maximize genset efficiency, creating a viable
alternative to traditional one-speed power platforms – variable speed gensets.
This game-changing improvement is made possible when the engine is freed from having to run
at a constant speed to produce a fixed output frequency (50 or 60Hz), and runs instead at the
most efficient speed and load for a given power output. The resulting operational efficiency runs
from a few percent to thirty-five percent, for average loads of eighty-five to ten percent.
This document describes our variable speed solution for efficiency improvement, how it works,
and why we believe it is the most competitive solution.
Why Variable Speed?
Innovus Power Figure 1 shows the typical efficiency, relative to full load, of a modern medium sized genset
(~200kW) operating at constant speed (lower blue line) and variable speed (upper red line).
100%
% Full Load Efficiency
90%
80%
70%
Constant Speed
60%
Variable Speed
50%
0%
20%
40%
60%
80%
100%
% Full Load
Figure 1 – Engine Efficiency vs. Load, Fixed and Variable Speed
If all power requirements were constant at maximum load, there would be no need for an
alternative, since the gensets would be continuously operating at peak efficiency. But the reality
is that gensets are called upon to provide a range of power requirements that significantly impact
performance.
For instance, consider a fixed-speed engine, operating at half load on average, would have an
efficiency decrease of around 10%; that is, it would use 10% more fuel to generate the same kW
energy as an engine operating at full load. At one third the load, the decrease is 25%. At onesixth the load, it’s 35%.
And these loads are not unusual in a significant number of applications. In the case of motor
start loads, for example, it is typical to size the genset 2-3x the maximum load, leaving the genset
operating well below half load on average. In addition, many industrial and environmental
control loads vary greatly between nearly zero load and full load. Even in the case of fairly
steady prime power, there is typically enough load variation to move the mean engine operating
point far enough that a few percentage points of efficiency are lost.
However, when an engine is allowed to run at the speed that is most efficient for its required
power output, as with a variable speed genset, the efficiency is nearly flat over a load range from
100% down to about 40%, and remains well above the fixed speed efficiency throughout the
operating range. This is shown with the upper red line in Figure 1.
Innovus Power The operating distinctions between fixed and variable speed translate into tangible results.
In Figure 1, the area between the red and blue lines represents the potential fuel savings from
variable speed operation. This is significant considering that in the lifetime of a fixed-speed
genset purchased today, diesel fuel will cost between 5-20 times more than the generation
equipment itself, a large, volatile cost in capital equipment investment decisions.
In addition, emissions from variable speed operations, which are directly tied to fuel use, are
reduced correspondingly. This is increasingly significant for regulatory reasons related to
government policy with regard to a carbon footprint and climate change.
There is also an important maintenance issue. Fixed speed engines that operate at light load and
high speed experience a significant loss of life. The low pressures keep the piston rings from
sealing correctly resulting in internal glazing and carbon buildup. This condition eventually
requires an engine rebuild far earlier than normal. In contrast, variable speed gensets operating at
lower speeds and higher torques, which maximize efficiency, also increases piston ring pressure
and minimizes internal glazing and carbon buildup, which extends engine life.
Finally a genset operating at fixed speed also implies that it cannot run at higher speed to get
additional power. However, a genset that operates below peak power most of the time and can
operate at higher (above rated) speed/power for limited periods can use a smaller, lighter, more
efficient engine. Since engine rating is typically for average power, applications that run both
above and below rated peak but maintain an average below rated power, will not see any
decrease in engine life.
Innovus Power Approach
A typical conventional genset configuration is shown in Figure 2. The engine runs at constant
speed to maintain the required output frequency. The genset field winding is controlled to
produce the required voltage.
Engine
(Fixed Speed)
Field Wound
Genhead
(90% efficient)
3-Phase AC
Figure 2 – Typical Fixed Speed Conventional Genset Configuration
The Innovus Power Variable Speed Genset configuration is shown in Figure 3.
Engine
(Variable Speed)
Permanent Magnet AC Active Rectifier DC
Genhead
with Boost
(98% efficient)
(96% efficient)
Inverter
with PFC
(96% efficient)
Figure 3 – Innovus Power Variable Speed Genset Configuration
Innovus Power 3-Phase AC
In the Innovus Power approach, the engine is now free to run at any speed, operating at the
optimum efficiency point (load and speed) for the required power output with the efficiency
advantages described earlier. The engine is also free to operate at higher speeds allowing more
power from a given engine. This means that a smaller, lighter, more efficient engine can be used
to meet peak power requirements.
The field wound genhead is replaced with a more efficient permanent magnet type. This is made
possible because the active rectifier now controls the variable voltage output. The active rectifier
provides both a voltage boost (since the genhead output can be well below the required output
voltage) and power factor correction. The output of the active rectifier is a fixed DC voltage and
the genhead runs at essentially unity power factor.
The internal inductance of the genhead is used for the voltage boost component to eliminate the
need for an expensive additional component. The unity power factor operation does not
fundamentally change the engine load. However, it does help improve genhead efficiency (or
decrease cost/size) because current magnitude and associated losses are lower.
The inverter (DC -> AC) converts the DC voltage to AC for loads that require it. It is an
optional module. An additional advantage of the inverter is that it can limit output current from
motor start and other transient loads. This means that the Innovus Power Genset required for
some applications can be much smaller (2-3x) than would be the case for a conventional genset.
In some cases only DC is required. A variable speed motor drive would be one example. This is
one of the significant advantages of the Innovus Power approach. Currently, a variable speed
motor drive would require an AC to DC rectification before the motor drive. With this
configuration comes an associated efficiency loss and cost for an unnecessary rectifier.
Figure 4 shows the efficiency gain of the Innovus Power variable speed system vs. a
conventional fixed speed genset. This data includes a smaller engine (which can go to the same
peak power as the conventional fixed speed genset it is compared to), genhead, active rectifier,
and inverter efficiencies and therefore represents the total genset efficiency improvement.
Innovus Power 35%$
30%$
Fuel%Savings%
25%$
20%$
15%$
10%$
5%$
0%$
!5%$
20%$
30%$
40%$
50%$
60%$
70%$
80%$
90%$
100%$
Mean.To.Peak%Load%Ra6o%
Figure 4 – Fuel Savings With Innovus Power’s Variable Speed Genset
Figure 5 compares the annual fuel savings to the incremental price for 200kW variable speed
genset. It shows that even at the lowest fuel costs the payback is less than a year for most
applications.
Innovus Power $80,000&
$60,000&
$40,000&
$20,000&
$0&
30&
40&
50&
60&
70&
80&
90&
100&
!$20,000&
Mean%To%Peak*Load*%*
!$40,000&
Annual&Fuel&Savings&($6/gal,&175&g/kWh,&8760&hours/year)&
Annual&Fuel&Savings&($4/gal,&175&g/kWh,&8760&hours/year))&
Variable&Speed&Generator&Price&Difference&(Fixed&Speed&25%&GM,&Variable&Speed&50%&GM,&1000&units)&
Figure 5 – Annual Fuel Savings With Innovus Power’s Variable Speed Genset
Key to the Innovus Power technological approach is the ability to control transient load.
When a Innovus Power variable speed genset is subject to an increase in load, all of its
components must work together to adjust to the new load condition. The turbocharged diesel
engine takes the longest to respond and requires more time for large load steps than to small
ones. The response time of the entire system, therefore, tracks that of the diesel engine and
depends strongly on the size of the load step.
In the example shown in Figure 6, at 10% load, the engine can deliver the required power at idle
speed. At time A, an additional 60% of the full rated load is suddenly connected. The engine
must accelerate to deliver the required power. Since it cannot do so instantaneously, the output
voltage drops below the set point.
Innovus Power 100%
C
voltage
load
50%
RPM
0%
Recovery time A
A
Recovery time B
B
Figure 6 – Transient Response
Because the genset always operates the engine with some power in reserve, smaller load steps
require little to no change in engine speed. In such cases, the active rectifier, and not the engine
dominate the response time. For example, at time B the engine accelerates only slightly to
maintain its reserve power, but is capable of supplying the load immediately
Diesel engines respond quickly to load changes at high speed, where maximum power is
available but fuel economy is lowest, and respond more slowly at low speed, where fuel
economy is highest. A balance must be made between fuel economy and transient response.
Innovus Power gensets adjust this balance in real time to provide the fuel economy benefit of
variable speed operation without sacrificing performance.
For instance, if a load anticipation signal is available, the controller can accelerate the engine
before the load is applied. This is shown in Figure 7, where a signal is supplied at time A. In
this case, Notice how the engine rapidly accelerates and stabilizes at 60% of its maximum speed
well before time B, when the load is connected; the amplitude and time of the voltage deflection
are considerably reduced. A small amount of supplemental energy storage, such as batteries or
capacitors may also be used to provide additional energy during transients.
Innovus Power 100%
voltage
RPM
50%
load
0%
Event duration
Recovery time
A
B
Figure 7 – Response With Load Anticipation Signal
Load Sharing
The conventional genset described in Figure 2 can share load with other gensets. However, the
process requires detailed analysis and costly additional controls. Typically, an additional system
controller manages the respective genset output. Each genset in the system is programmed (after
detailed system analysis) with a droop (decreasing voltage with increasing power) for power
sharing and a speed droop (decreasing frequency with increasing power) for frequency matching.
Additional control (e.g. cross-current compensation, kW and KVAR load sharing) and safety
features are also required. This is generally only practical for larger fixed systems or systems
that will remain in place for a long period.
The Innovus Power genset described in Figure 3 can load share in three ways. The DC outputs
have voltage droop (decreasing voltage with increasing output) and will share naturally for
systems only requiring a DC output. Multiple DC gensets can load share on a DC buss and then
be connected to a single DC->AC inverter.
The second possibility is that the gensets are grid connected. In this case, the inverter
automatically locks on to the grid frequency and maintains its output power at the commanded
level with no additional analysis or controls and cost. The third possibility is for AC off-grid
applications. One genset is set to be a master and produces the required frequency
(isosynchronus control). The other gensets automatically lock on to the master frequency and
use a voltage droop control to provide stable load sharing. In essence, the load sharing is plugand-play making it practical even for novice users in the field.
This easy implementation of load sharing reduces the number of genset sizes required in the
product portfolio. Multiples of one size can easily be used for different load requirements. This
approach improves logistics and maintenance for customers since there are many fewer genset
Innovus Power types to be managed.
Innovus Power Genset Architecture
Innovus Power gensets employ a modular architecture as shown in Figure 8 with time-critical
tasks handled by dedicated electronic modules.
A system controller coordinates the operation of these modules, reports system status, and
accepts commands via a touch screen or other user interface. This modular architecture
minimizes long wire runs in the chassis and improves field serviceability. A communications
port on the front panel allows multiple units to be connected together. This interface also
provides diagnostic capability, software upgrades, and remote changes to control parameters for
performance optimization.
The system controller provides zero-droop load balancing in multiple-genset systems and allows
any number of units to be started and stopped from a single location. For added safety, a
separate ground fault detection (GFD) module monitors the isolation integrity of the voltage
thereby displaying an alarm if an insulation fault is detected.
•
•
•
•
•
Life Optimizer
Performance Optimizer (efficiency, emissions, transient response)
System controller
Diesel Engine
(variable speed)
Generator
(permanent
magnet)
Active
rectifier
Inverter
Fleet management
Service management
Remote diagnostics
Remote configuration
Software updates
3-Phase AC
Constant Hz
DC
Conventional Technology
Use Of Proven Technology
New Capabilities
Figure 8 – Innovus Power Genset Architecture
Engine
Conventional AC gensets must operate at a speed dictated by electrical frequency, typically 1500
or 1800 RPM. A variable speed genset can run at higher speeds to produce more power with the
same size engine or can use a smaller engine to produce equivalent power output.
Innovus Power prefers to incorporate advanced electronically controlled diesel engines into the
company’s Gensets. Electronic engine controls provide greater performance, lower emissions,
and a higher level of system integration than mechanical controls.
Genhead
Innovus Power The genhead is designed to match the torque characteristics of the engine. Diesel engines
operate most efficiently at maximum torque and below maximum speed. Most engines provide
more torque and greater fuel economy at intermediate speeds than at maximum speed. For the
genhead to operate most efficiently, the genhead must be somewhat oversized to take advantage
of this extra torque.
Other weight savings due to the greater efficiency of the permanent magnet genhead offsets the
impact on total weight over conventional designs. Larger genheads are generally more efficient
than smaller ones and require less cooling due to their lower resistance and core losses. Further,
induction and wound field synchronous genheads dissipate considerable power in the rotor and
field windings, respectively, while permanent magnet synchronous machines do not. The lower
power losses of permanent magnet genheads allow the use of liquid cooling, which reduces the
weight of the housing.
Permanent magnet genheads produce a voltage, called back EMF, during rotation. In Innovus
Power genheads, an active rectifier drives current through the stator in synchronization with the
back EMF to produce DC output. The shape and frequency of the back EMF are critical to
achieving high efficiency and ripple-free DC output.
The genhead is designed to produce as clean a sine wave as possible with minimal harmonics;
the wye connection of the windings eliminates multiples of the third harmonic, and the pitch of
the rotor and stator reduces other harmonic content to yield a clean waveform.
Innovus Power gensets are engineered for years of reliable operation under extreme conditions.
They do not require brushes or slip rings, which are common failure points in other designs.
Further, the electrical insulation is designed for long life at high temperatures using Class H
materials.
Power Electronic Module (active rectifier and inverter)
The active rectifier converts the output of the genhead into DC. It consists of two parts:
1. A liquid cooled power electronic module
2. A microprocessor-controlled electronics package to drive the power electronics
The power electronic module contains six IGBTs connected to a capacitor bank. Lowimpedance polypropylene film capacitors are mounted to a laminated bus bar structure designed
for minimal inductance. Filters are added as necessary to reduce voltage ripple or conducted
emissions to within application limits. If the application requires AC output, the power electronic
module also includes an inverter.
The electronic controller is housed in a sealed, field-replaceable module adjacent to the rectifier.
The module contains a microprocessor, signal conditioning circuits, an optically isolated
interface to drive the power electronics, and a communications interface. The controller receives
measurements of the phase current and rotor angle in order to synchronize the phase current to
the genhead and adjusts the amount of current to regulate the DC output. To avoid overloading
Innovus Power the engine during abrupt load changes, the software limits the winding current according to
parameters received from the engine controller.
System Controller
The system controller monitors the engine and rectifier control modules through a dedicated
communications bus. It governs the start-up sequence of the engine, applies starting aids as
required in cold weather, and limits cranking time to prevent damage to the starter. It also
monitors oil temperature, level, and lifetime; fuel levels; coolant level and temperature; and
stator temperature, displaying alarms as necessary. The system controller coordinates the
operation of the engine and active rectifier within the genset and among other gensets, using
efficiency maps of each component to optimize system efficiency while providing droop-less
load sharing across multiple units connected in parallel.
Conclusion
The Innovus Power variable speed genset offers game-changing advantages over conventional
genset technology. It yields significant fuel savings in nearly all applications and higher than
35% in some applications. Its incremental cost pays back from fuel savings in less than a year
for most applications at $4/gal fuel. The payback would be measured in months when fuel costs
are based on “cost delivered” in remote military locations. The Innovus Power genset
technology also lowers emissions and is smaller and lighter than any of its competitors. It offers
plug and play load sharing unlike any other product in the industry.
For its versatility, fuel savings, emissions, size and weight reductions over conventional gensets,
and companion improvements in service life and maintenance requirements, the Innovus Power
variable speed genset represents a compelling, but technologically grounded next step in genset
design and capability.
Innovus Power