3. GE Proprietary Information
Need to Know Interface Data
• Rectifier Power
– Remember the input power of a UPS is different than the output power, due to its efficiency,
losses and battery re-charge. For example a 150KVA Signature UPS has an input power rating
of 162KVA.
– Some UPS do not have a soft start feature. In this case the generator will have to be sized to
accept all the inrush current of the UPS during energization.
• Input Harmonics
– A standard 6 pulse rectifier generates input current harmonics on the order of 25%-30% these
are reflected back to the source and can generate voltage harmonics, which can make it difficult
for the generator to regulate voltage properly. These current harmonics can also overheat the
alternator windings in the generator.
• Sub-transient Reactance “Xd”
– It is best for a generator to have as low a sub-transient reactance as possible. This figure directly
relates to the voltage distortion that the generator will have to cope with. It is usually given as a
percentage. A good Xd is something around 8%-12%
• Total Load on Generator
– Be sure to take into account other loads that may be on the generator apart from the UPS, such
as air conditioning and lighting. Also if there are any pf correction capacitors in the system these
can intensify voltage distortion and need to taken into account. The line impedance of the input
cabling can also be a factor in the amount of voltage distortion seen by the generator.
4. GE Proprietary Information
Basic Generator Sizing
• Quick rule of thumb
– Generator size = 2.5 times UPS rating with no filter
– Generator size = 1.5 times UPS rating with input filter
• Generator sizing calculation
PG = PR x C x Xd / d
– PG is the Generator power;
– PR is Rectifier input power;
– Xd is the Sub transient Reactance of the generator;
– d is the acceptable THD voltage distortion;
– C is a constant value which is calculated by the number of Rectifier pulses and the total input line Impedance of
the electrical system.
– Typical values: for 6 pulse Rectifier C= 1.3 to 1.6
– for 6 pulse rectifier with input filter C= 0.8 to 1
– for 12 pulse Rectifier C= 0.8 to 1
Example: UPS 100 kVA; Acceptable voltage distortion 6%; Sub transient Reactance Xd=12%
– With 6 pulse Rectifier; PG= (110 x 1.3 x 0.12)/0.06 = 260 kVA
– With 6 pulse Rectifier and filter; PG= (110 x 0.8 x 0.12)/0.06 = 176 kVA
5. GE Proprietary Information
• In a system where a generator is already in place and a UPS size
has been determined it is possible to calculate the need for an
input filter based on the Voltage Distortion allowed by the
generator.
d = ( PR x C x Xd ) / PG
Example: UPS 300 kVA; EG 450 kVA; Xd = 0.12
d = (330 x 1.3 x 0.12) / 450 = 11% with 6 pulse Rectifier
d = (330 x 0.8 x 0.12) / 450 = 7% with 12 pulse Rectifier.
If a smaller voltage distortion is required it is necessary to install
filters.
It is generally good practice to include a 5th harmonic filter with
all generator applications.
Deciding if you need a 5th Harmonic Filter
6. GE Proprietary Information
The GE Advantage
• Soft Start
– The GE UPS has a soft start feature which slowly ramps the rectifier into its performance band.
This limits inrush currents that can cause problems with generator regulation, and also reduces
the need to over size the generator. The time window that the ramp up takes place is adjustable
through the UPS parameters up to 30 seconds.
• Adjustable frequency window and phase angle detection.
– The GE UPS can be fine tuned for the generator it will be interfacing with through its parameters.
If the generator has particularly poor frequency regulation the window can be widened to allow
the UPS to accept larger frequency fluctuations. If loads other than the UPS affect the utility
phase angle that the UPS is trying to track then the tracking window can also be adjusted to
allow the UPS and generator to stay in sync.
• Battery recharge disconnect
– If the utility fails and the UPS is forced to go on battery for a time before the generator is started,
the UPS can be set through its parameters to inhibit battery recharge once the generator comes
on line. This is advantageous because generator power is expensive and it would be useful to
recharge large battery systems with the electrical mains and not the generator.
• Programmable parameters
– There are other parameters that can be programmed from the front display that will reduce
UPS/Generator compatibility issues. Most of these serve to adjust the way that the UPS
interprets the generator signals and avoid erroneous alarms.
7. GE Proprietary Information
Active Filter
• 5th Harmonic Cancellation
– The optional filter is designed to cancel the current
distortion that is most present on the 5th harmonic.
• Power Factor Correction
– The input filter also raises the input pf to approximately 0.93
with addition of capacitors. This will allow the generator to
operate more efficiently delivering more real power.
• Automatic Delay and Disconnect
– If the UPS is energized while a generator is supplying the
source voltage the filter can be automatically taken out of
the circuit for a time to allow the generator and the UPS to
more easily sync together. Also, if the load drops below the
30% the filter is automatically removed from the circuit for
more efficient operation.
THD without filter
THD after filter
8. GE Proprietary Information
Summary
• The interfacing of a generator with a UPS is complex and requires special consideration of
many factors. When problems arise it is difficult to assign responsibility. The generator
manufacture may say it is a problem with the UPS load and the UPS manufacturer may say
that the system is working perfectly and that it is the generator which is unstable.
• Many customers will use generators to reduce the investment required for large battery
systems. In this case the required battery backup time needs only to be long enough to get the
generator on line. 5-10 minutes should compensate for the difference in generator starting
systems.
• The adjustable features available through the parameters allow the GE UPS to be fine tuned to
specific generator characteristics reducing possible interface issues and the time it takes for
the UPS and generator to couple together, this in turn reduces the battery runtime that is
required.
• The design of the GE UPS with adjustable Soft Start reduces the need for extensive generator
over sizing.
• The optional input filter does more than just reduce harmonics. Its programmable features
allow it to more easily interface with a generator further reducing UPS/Generator compatibility
issues and increasing efficiency.
