VYPER
™
VARIABLE SPEED DRIVE
INSTALLATION - OPERATION - MAINTENANCE
100-210 IOM (
JUL
09)
Page 12
HARMONIC FILTER
The VSD system may include an harmonic filter designed to
meet the IEEE Std 519 -1992, “IEEE Recommended Practices
and Requirements for Harmonic Control in Electrical Power
Systems”. The filter is offered as a means to “clean up” the input
current waveform drawn by the VSD from the power mains,
thus reducing the possibility of causing electrical interference
with other sensitive electronic equipment connected to the
same power source.
Figure 3 is a plot of the typical input current waveform for the
VSD system without the harmonic filter when the system is
operating at 50% load. Figure 4 is a plot of the typical input
current waveform for the VSD system with the harmonic filter
installed when operating at the same load conditions. The
plots show that the input current waveform is converted from
a square wave to a fairly clean sinusoidal waveform when the
filter is installed. In addition, the power factor of the system
with the harmonic filter installed corrects the system power
factor to nearly unity.
The power section of the Harmonic Filter is composed of four
major blocks: a precharge section, a “trap” filter network, a
three phase inductor and an IGBT Phase Bank Assembly.
See the elementary wiring diagram in Figures 5A and 5B for
wiring and component reference
The Precharge Section
is formed by three resistors (11RES
- 13RES) and two contactors, the precharge contactor (2M),
and the supply contactor (3M). The precharge network serves
two purposes, to slowly charge the DC link filter capacitors
associated with the filter Phase Bank Assembly (via the
diodes within the IGBT modules Q13-Q18) and to provide
a means of disconnecting the filter power components from
the power mains. When the drive is turned off, both contac-
tors are dropped out and the filter phase bank assembly is
disconnected from the mains.
When the drive is commanded to run, the precharge resistors
are switched into the circuit via contactor 2M for a fixed time
period of five (5) seconds. This permits the filter capacitors
in the phase bank assembly to slowly charge. After the five-
second time period, the supply contactor is pulled in and the
precharge contactor is dropped out, permitting the filter phase
bank assembly to completely charge to the peak of the input
power mains. Three power fuses (11FU -13FU) connect the
filter power components to the power mains. Very fast semi-
conductor power fuses are utilized to ensure that the IGBT
modules do not rupture if a catastrophic failure occurs on the
DC link of the filter phase bank assembly.
The “Trap” Filter
is composed of a series of capacitors (C84-
C92), inductors (4L-6L) and resistors (16RES-18RES). The
“trap” filter acts as a low impedance for a range of frequencies
centered at the PWM switching frequency of the filter (20 kHz).
The purpose of the trap is to block currents at the switching
frequency of the filter from getting onto the power mains.
The Three-Phase Inductor
provides some impedance for
the filter to “work against”. The inductor effectively limits the
rate of change of current at the input to the filter to a reason-
able level.
T
he IGBT Phase Bank Assembly
is the most complicated
power component in the harmonic filter. The purpose of the
assembly is to generate harmonic currents required by the
drive’s AC to DC converter so that these harmonic currents are
not drawn from the power mains. The phase bank is composed
of a series of IGBT modules (Q13-Q18) mounted to a liquid-
cooled heatsink, a filter capacitor “bank” (C67-C76) and an
IEEE 519 Filter Gate Driver board (031-01786) which provides
the On and Off gating pulses to the IGBT’s as determined by
the 519 Filter Logic board. In order to assure an equal shar-
ing of the voltage between the series connected capacitors
on the filter bank, “bleeder” resistors 14RES and 15RES
are connected across the banks. In order to counteract the
parasitic inductances in the mechanical structure of the phase
bank, the filter incorporates “laminated bus” technology and
a series of small film capacitors (C77-C83). The technology
used is identical to that used in the VSD’s DC to AC inverter
section of the drive.
Various ancillary sensors and circuit boards are used to convey
information back to the harmonic filter logic board. The follow-
ing list describes some of these components.
• A
thermistor temperature sensor, RT5, is mounted onto the
liquid-cooled heatsink to provide temperature information.
• Current Transformers 6T and 7T sense the input current
drawn by the VSD’s AC to DC converter.
• DC Current Transformers (DCCT1 and DCCT2) sense the
current generated by the harmonic filter.
• The Line Voltage Isolation Board (031-02022) senses the
input voltage to the system, steps the voltage down to a safe
level and provides isolation between the Filter Logic board
and the power mains.
• The Bus Isolation Board (031-01624) incorporates three
resistors to provide a “safe” impedance between the DC filter
capacitors located on the phase bank assembly and the filter
logic board. The bus isolation board provides the means to
sense the positive, midpoint, and negative connection points
of the filter’s DC link.
