SC9000E预充电系统应用说明说明书

SC9000 pre-charge system
Aaron H VanderMeulen Application Engineer
Irving Gibbs Principal Engineer
Doug Halamay Power System Engineer The SC9000E pre-charge system provides many benefits: 1) eliminates drive inrush current, 2) simplifies protection and coordination, and 3) provides a possible reduction of incident energy and PPE level at the drive. The intent of this application note is to inform end users of the benefits of the pre-charge system and how it operates. Also, power systems engineers can use this as a guide when performing protective relaying coordination and arc-flash analysis where the
SC9000 is installed.
Eliminating inrush current
Many large horsepower adjustable frequency drive
(AFD) systems incorporate a DC bus pre-charge,
specifically those of voltage source inverter
(VSI) topology. VSI topologies utilize a single or
distributed capacitive DC bus. The SC9000 has a
single DC bus. An uncharged capacitive circuit is
seen as a short circuit when line voltage is first
applied. Applying line voltage to the AFD diode
rectifier (converter) with an uncharged DC bus
results in extremely high inrush current. This inrush
current can stress and, in some cases, cause diode
failure. A solution is to “pre-charge” the DC bus
prior to application of line voltage. Once the DC bus
is charged to a nominal level, applying line voltage
reduces inrush current. Estimated DC bus voltage
is 1.414 times line voltage (example: 4160 V line
voltage equates to 5883 Vdc).
The SC9000 utilizes an integral transformer that
provides electrical isolation from the line and
multi-pulse configurations for low line harmonics.
When using a pre-charge, the capacitive DC bus
is charged prior to application of line voltage;
however, the isolation transformer inrush currents
must be addressed. Phase shifting isolation
transformers are designed with multiple secondary
circuits, low impedance, and high efficiency.
These design requirements result in significant
inrush (magnetizing inrush) currents. Feeder
and upstream protective relaying coordination
must take into account the magnitude and
duration of AFD transformer energization. Field
measurements have shown the inrush current
range from 10x to 20x the nominal full load input
amps of the transformer. Calculation of exact
magnetizing inrush current is not trivial and
depends upon the transformer design.
Low horsepower, low-voltage AFDs typically do
not use a pre-charge system because the DC bus
capacitance is relatively small. Large horsepower
low-voltage AFDs require DC bus pre-charge
to address the large inrush currents. One
advantage is that low-voltage drives do not
require or use isolation transformers, excluding
“Clean Power” varieties.
Typical pre-charge systems:
1. DC bus charging circuit only (Figure 1)
2. Current limiting reactor with bypass
contactor (Figure 2)
Figure 1 illustrates a fully integrated AFD system
with fused input contactor and non-load break
isolation switch as a feeder. The AFD is comprised
of isolation transformer, diode rectifier, DC bus
capacitor and insulated-gate bipolar transistor
(IGBT) based inverter. Figure 2 illustrates a fully
integrated AFD system with the same feeder;
however, a current-limiting reactor and a bypass
contactor is added.
Pre-charge of the DC bus protects the diode
rectifier, but does not eliminate transformer
inrush. Utilizing a current-limiting reactor reduces
inrush current (both transformer and capacitor
charging); however, an additional medium-voltage
rated contactor is needed to bypass the reactor.
This adds complexity, equipment footprint, and
cost. It should be noted that a current-limiting
reactor pre-charge may have a limited number
of consecutive “starts”.
The SC9000 utilizes a “soft-mag” pre-charge
system. The isolation transformer is magnetized
and the DC bus capacitors are charged at the
same time. An integral three-phase step-down
control power transformer is tied into the
secondary windings of the isolation transformer.
The soft-mag impedance limits the current and
magnetizes the isolation transformer in phase
with the line voltage. When the input contactor
is closed, there is zero inrush current on the line
side of the AFD. The soft-mag is also used for
enclosure cooling power, making it dual purpose.
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Figure 1. DC bus charging circuit only
Figure 2. Current-limiting reactor with bypass contactor
Figure 3. DC bus pre-charge and soft-mag pre-charge method A medium-voltage drive rated 4160 V , 2500 hp with 304 A full load input current was tested using the DC bus pre-charge and soft-mag pre-charge method (Figure 3).
Measurements taken from a DC bus pre-charge is shown in Figure 4. The top portion of the oscilloscope wave capture is the input current with the peak recorded current over 3500 A. The bottom portion is several microseconds after the contactor closes, highlighting high frequency ringing. The high frequency component can cause resonance due to internal winding capacitance. Internal transformer resonance can cause severe transformer damage.Measurements taken with a soft-mag pre-charge is shown in Figure 5. The oscilloscope waveform capture is of three-phase voltage input and one phase input current (green trace). The input current is significantly reduced to approximately 200 A.
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Figure 4. Measurements taken from a DC bus pre-charge
Figure 5. Measurements taken with a soft-mag pre-charge
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Simplifying protection and coordination
Upstream protective relay coordination can be optimized by
simplifying current pick-up, time delay, and trip curves. Traditional protective relay coordination may include long-time, short-time, and instantaneous curves.
Figure 6. Example of a single utility feed, step-down distribution transformer to a 4.16 kV bus
The one-line diagram (Figure 6) is an example of a single utility feed, step-down distribution transformer to a 4.16 kV bus. The bus 2 has a single AFD, but the same principle applies for multiple AFDs in a line-up or with separate feeders.
Example time current curves are represented in Figure 7. If a short circuit occurs within the zone of protection, the total clearing time depends upon if the fault is upstream or downstream of the drive main fuses. The breaker protective relay coordination curve is shown in red.
It is shown that the drive main fuses are coordinated to allow for the isolation transformer inrush (blue). To maintain proper breaker coordination, the protection curve must also allow for the isolation transformer inrush. This eliminates nuisance tripping, but sacrifices coordination; this is represented with overlapping of the fuse and protective relay curves.
With soft-mag pre-charge, the inrush current is eliminated and therefore the breaker protective relay time-current curve is
configured to a definite time trip characteristic only. The current pick-up is set at 1.25 times nominal input current, which accounts for line voltage fluctuations and temporary overload conditions. Revised time current curves are represented in Figure 8.
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Figure 7. Time current curve
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Figure 8. Time current curve
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An arc-flash evaluation at the line-side of the AFD is simulated with power system analysis software applying the IEEE T 1584 preferred method.
Comparing the two arc-flash calculations, the soft-mag pre-charge reduces the incident energy level from 8.3 cal/cm 2 to 1.9 cal/cm 2. The available short-circuit current has not changed, but the tripping time has been drastically reduced and therefore the recommended PPE is reduced.
Occupational Safety and Health Administration (OSHA) provides guidance in 29 CFR 1910.132(d)(1) regarding employers to address if hazards are present that may necessitate the use of PPE. The methodology outlined assists power system engineers in providing upstream protective relay coordination for SC9000 medium-voltage drives and provides engineering controls to possibly reduce the available incident energy at the line-side of the drive.
Summary
The SC9000 medium-voltage drive incorporates a soft-mag pre-charge system. This system provides the following:1. Eliminates drive inrush current.2. Simplifies protection and coordination.
3. Provides a possible reduction of incident energy and
PPE level at the drive.
Eaton
1000 Eaton Boulevard Cleveland, OH 44122 United States
© 2022 Eaton
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Printed in USA
Publication No. AP020004EN / Z26333 April 2022
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of their respective owners.
SC9000 pre-charge system
Application Note AP020004EN Effective April 2022。