04-05-2012, 02:40 PM
Assessment of Ride-Through Alternatives for Adjustable-Speed Drives
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INTRODUCTION
THE application of adjustable-speed drives (ASD’s) in
commercial and industrial facilities is increasing due
to improved efficiency, energy savings, and process control.
However, ASD’s are often susceptible to voltage disturbances,
such as sags, swells, transients (e.g., due to capacitor switching),
and momentary interruptions (outages). According to
survey reports, voltage sags of 10%–30% below nominal for
3–30-cycle durations account for the majority of power system
disturbances, and are the major cause of industry process
disruptions [1].
MODIFICATIONS TO EXISTING ASD TOPOLOGIES
Existing drive topologies can be modified to achieve a
higher level of immunity to line disturbances. These include
adding more capacitors to the dc bus, ride-through using load
inertia, operating ASD’s at reduced speed and/or load, and
using lower voltage motors. An important distinction between
each of the possible ride-through approaches is their ability to
provide full-power (full speed and torque) ride-through, which
is required by many applications.
MORE ADVANCED HARDWARE MODIFICATIONS
A. Boost Converter Ride-Through
A boost converter can be used to maintain the dc-bus
voltage during a voltage sag, and can either be integrated into
new drives between the rectifier and the dc-link capacitors or
retrofitted as an add-on module, as in Fig. 2 [10]. The addon
module is used to retrofit existing drives with ride-through
capabilities, or for multiple drives with a common dc bus,
such as synthetic fiber drives. During a voltage sag, the boost
converter will sense a drop in the dc-bus voltage and begin to
regulate the dc bus to the minimum voltage required by the
inverter (i.e., 585 V, which is user adjustable). In the case of
a retrofit where a boost module is added to an existing ASD,
proper coordination of fault protection logic is necessary.
Advantages
• Clean input power in steady state, unity power factor, and
compliance to IEEE 519 distortion limits are provided.
• The active rectifier provides a regulated dc-bus voltage,
hence, it is self correcting under voltage sags. A suitable
rectifier derating is necessary to provide full-power ridethrough
under a sag.
• Power flow in both directions enables regenerative braking.
This feature could add to improved efficiency in some
applications.
Disadvantages
• An ASD with an active PWM rectifier is nearly equivalent
to two diode rectifier ASD’s in cost.
• The ASD package is larger in size, since, in addition to
the active rectifier hardware, three input filter inductors
become necessary.
• The active rectifier operates the ASD with higher dc-link
voltage; this results in higher differential mode at
the motor terminals. Also, due to two PWM insulated gate
bipolar transistor (IGBT) inverter stages, the commonmode
and EMI are higher.