Xylem IM167 R8 Aquavar CPC (Centrifugal Pump Control) - Manual (Page 12)

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if transformer impedance is greater than 3%. The transformer manufacturer may provide
derating for non K Factor rated transformers to operate at the drive produced K Factor levels.
Other transformer configurations are acceptable. Three single phase transformers can be
used if they are identical for phase to phase symmetry and balance. A wye connected
primary neutral should never be grounded. Great care should be taken with delta primary
delta secondary configurations. Any lack of phase to phase symmetry could result in
circulating currents and unacceptable transformer heating.
WARNING! Never use phase converters with drives as nuisance tripping and possible
damage may occur. Size the drive for single phase input using 50% derate factor.
WARNING! Use of “Open Delta” 230 volt power systems should be sized using the
50% derate factor. Consult factory.
Sometimes drive input isolation transformers are specified to deal with one or more of
the following issues:
1. Short Circuit Protection: Input transformers are sometimes used to provide impedance
to reduce the available short circuit current to levels that the input clearing devices,
such as fuses or circuit breakers, are rated to handle. Line reactors can perform this
impedance function much more cost effectively.
2. Transient Protection: Input transformers are sometimes used to provide transient
surge impedance. All the Aquavar CPC drives have capacitors and MOV’s (Metal Oxide
Varistor transient protectors) providing 120 to 360 joules, line to line and line to ground
transient protection. Isolation transformers are not required for this protection within
those energy levels. Additional distribution transformer primary transient surge arrestors
may be required if the potential transient energy reflected to the drive exceed those
levels. MOV’s are rated to handle high levels of one shot transient energy. MOV’s are not
meant to handle continuously recurring transients. A problem of continuously recurring
transients should be corrected before connecting a drive.
3. Harmonic Mitigation: Input transformers are sometimes used to provide impedance to
reduce the harmonic currents generated in the drive. Line reactors can perform this
function much more cost effectively.
4. Power Factor Capacitor Isolation: Input transformers are sometimes used to provide
impedance to isolate drives from line connected power factor correction capacitors.
PWM drive inputs do not require power factor correction capacitors as drive power
factors are generally greater than 92% and cannot be significantly improved with
power factor correction capacitors which only correct for fundamental. However drives
should be isolated from power factor correction capacitors by about 3 to 6% additional
impedance with respect to the drives. Line reactors can perform this function much more
cost effectively than isolation transformers. ABB drives have either an internal 3% line
reactor or an equivalent 3 to 5% bus reactor.
5. RFI/EMI Mitigation: Neither input isolation transformers nor line or bus reactors provide
good high frequency filtering although an isolation transformer with a static shield will
provide some RFI mitigation. If RFI/EMI mitigation is required, an RFI/EMI filter mounted
inside the drive should be used together with all the proper wiring and grounding
techniques. Some RFI/EMI filters may operate only on a power source with a grounded
neutral. Establishing a local neutral ground may require the use of an input isolation
transformer.
INTRODUCTION