cccff> « back to results for ""
Below is a cache of http://www.solidstatecontrolsinc.com/techhpapers/pdfs/sizingupsnonlinear.pdf. It's a snapshot of the page taken as our search engine crawled the Web.
The web site itself may have changed. You can check the current page or check for previous versions at the Internet Archive. Yahoo! is not affiliated with the authors of this page or responsible for its content.
6L]LQJ$8366\VWHP )RU1RQ/LQHDU/RDGV
6L]LQJ$8366\VWHP
)RU1RQ/LQHDU/RDGV
SOLIDSTATE
CONTROLS, INC.
Solidstate Controls Incorporated
875 Dearborn Drive
Columbus, Ohio 43085
Tel : (614) 846-7500
Fax: (614) 885-3990 SOLIDSTATE CONTROLS, INC.

6L]LQJ $ 836 6\VWHP )RU 1RQ/LQHDU /RDGV
-897,.9
The safe and efficient control of a critical process such as a recovery boiler system has been made possible by
Distributed Control Systems (DCS). These DCS systems often require uninterruptible AC voltage which is provided
by static (electronic) inverters.
The insertion of an inverter, which is a limited capacity device between the DCS and the utility power, requires some
knowledge of the input power requirements of distributed control system's internal power supplies.
Examination of the current waveform of a modern DCS would reveal a wave shape much like the one shown in
)LJXUH 
. The current is not sinusoidal and is not continuous. The peak current is much greater than the RMS (root-
mean-square) value, often 2.5 times as large. This kind of non-linear current is characteristic of the electronic AC-
DC power supplies used in many DCS systems.
Peak
RMS
Non-Linear Loads
Definition of Crest Factor
Crest Factor = peak
rms
Crest Factor (CF)
- The ratio of the peak value
to its RMS value
- For a sine wave the Crest
Factor is 1.414
)LJXUH 
Over the past 20 years, AC-DC power supplies have evolved from ferroresonant to series-regulator and finally to
switch-mode designs. Table "A" highlights some of the characteristics of the three types of power supplies.
The oldest design, the ferroresonant, uses a large input magnetic regulator. By today's DCS packaging standards,
the ferroresonant power supply is big and heavy. There are still some control systems manufacturers that use
ferroresonant supplies because of their reliability and low input harmonic current distortion.
The series-regulator design uses an input isolation transformer coupled with a transistorized voltage regulator stage.
Poor conversion efficiency and narrow input range have restricted its use.
The third type of power supply, the switch-mode (SMPS) has the advantage of the smallest size and lightest weight
because the line frequency magnetics have been eliminated. (Refer to
)LJXUH 
) The reduction in the input inductive
reactance and the insertion of a large capacitor together has produced an extremely compact but extremely non-
linear device. Since the SMPS is the dominant power supply in many DCS systems, its input current characteristics
must be understood when sizing an inverter. SOLIDSTATE CONTROLS, INC.

PWM
control
100kHz
oscillator
120 volts
60Hz
C
)LJXUH 
Switch Mode Power Supply (extremely non-Linear)
There are four (4) important factors that must be carefully considered whenever inverters are used with extremely
non-linear loads such as switch-mode power supplies (SMPS). These factors are:


Crest Factor (Published Vs Actual)


Repetitive & Non-Repetitive peak load currents


Harmonic Voltage Distortion


Inverter Technology
&UHVW )DFWRU 3XEOLVKHG YV $FWXDO
Many distributed Control System (DCS) manuals make reference to a ratio often called "Crest Factor" which is
simply the ratio of the peak current value divided by the root-mean-square (RMS) value. Often in these manuals the
user is cautioned to oversize the inverter based on published crest factor data for the various control modules that
comprise the DCS system. This approach if not used with caution will invariably lead to oversized inverters.
Suppose we construct a hypothetical distributed control system that will require (20) twenty control modules, with
each control module using one switch-mode power supply. Let us further suppose that in our hypothetical system,
the power supply is a 200W model as shown in Table "B". (Table "B" is a typical listing of the SMPS input
characteristics)
This supply can produce 200 watts of DC output power, has a universal 120/240 V, 50/60 Hz input, and is designed
to maintain its DC output voltage for 10 milliseconds with a total loss of AC input. This power hold-up time is supplied
from an internal 470 UF or VF electrolytic capacitor (Figure 2 component "C")
Since our DCS system will use twenty (20) of these supplies in its overall control scheme, one might be tempted to
multiply the 432 VA input demand times 20 and add "fudge factors because of the large crest factor (Table "B",
column 5B).
Very often inverters are over-sized because their rated crest factor (typically 3.0) is less than published power supply
data. Table B indicates a power supplies crest factor of 3.4.
The assumption, of course, is that the crest factor of the switch-mode supply is a constant value. Table "C"
summarized actual values taken on a 10 kVA ferroresonant inverter with a simulated switch-mode load. The load of
8.43 kVA is very close to our hypothetical load of twenty (20) 200W power supplies. In Table "C", notice that the load SOLIDSTATE CONTROLS, INC.

crest factor has decreased from 3.4 to 2.3. This reduction in crest factor is a result of a change in the source to load
impedance ration (Zs/Zl, see figure 3). An example will clarify the concept:
Suppose that when the original performance data in Table "B" was measured, a 3 kVA transformer with a 5%
impedance was used to provide power to a single 200 watt supply.
The source-load impedance ratio in this case would be:
Zs/ ZK =.24/.33= 1/138
Where Z= E2VA
If we were to add (5) five more 200w supplies for a total of 6 units, the new ratio would be:
Zs/ ZL = .24/5.5 =1/23
Z
S
V
S
Z
L
)LJXUH 
Switch Mode Power Supply
Published power supply data rarely takes this effect into account and thus actual crest factors will be lower.
The reduction in crest factor is a predictable and repeatable result whenever switch-mode power supplies are used
with power sources that have inductive reactance in series with the switch-mode power supply. In Table "C", the
inverter's output reactance reduced the load crest factor. The reduction in crest factor is accomplished by changing
the shape of the switch mode's input current waveform to a wider shape with less amplitude. The area under the
current curve tends to remain constant, but the harmonics, particularly the 3rd and 5th, are attenuated.
Measurements performed on 10 & 30 kVA ferroresonant inverters with simulated switch-mode loads (see tables "D"
&"E") show that over a load range of 25-100% the load crest factor values ranged from 2.2 to 2.5.
The important point to be emphasized from the data in tables "D" & "E" is that ferroresonant inverters do not need to
be oversized because of the high crest factor associated with switch-mode power supplies. Crest factor is
dependent on the application and must be used with caution as an inverter sizing criterion.
Again, referring to table "C", the peak current has been reduced from a predicted value of 12.2 x 20 =224 A (Table
"B" column 4B) to an actual value of 156A. (Table "C" column 4C) but even these reduced values of peak current
can be a problem for some inverter technologies that use linear magnetics. SOLIDSTATE CONTROLS, INC.

Inverter
Bypass
Source
AC Load
AC Load
AC Load
Switch Mode
Power Supplies
Static
Switch
Batteries
)LJXUH 
3HDN &XUUHQW (IIHFWV
)LJXUH 
is a simple block diagram of an inverter with a static switch used in critical control applications.
This topology is commonly called an "on-line" system because the inverter normally supplies the critical load.
Between the inverter and the critical load is an electronic switch (static) that will transfer the critical load to a fail-over
source (static bypass), should the load current demand exceed the inverters rated capacity (or should the inverter
fail)
If the static switch is designed for sinusoidal current with a crest factor of 1.414, then a non-sinusoidal current with a
crest factor of 2.4 will cause premature load switching to bypass. Static switch transfer sensing for overcurrent is
best accomplished with true RMS values rather than peak current values.
For example, if the 10kVA system (Table "C" data) had used a sinusoidal, peak sensing current design the
connected loads peak current of 156A (column 4C) would have been 11% over its static switch transfer point,
assuming a 120% overload capacity. True RMS sensing rather than peak sensing is a very important feature in a
well-designed system to prevent premature static switch operation.
| Peak
(this is the
specified value)
Note that the inrush
diminished as time progresses
Power is applied to the unit here
T
)LJXUH 
Inrush current that occurs when the power is first applied to a unit. It is