Rockwell Automation 7000 PowerFlex Medium Voltage AC Drive (B Frame) - ForGe Control - Manual (Page 35)

Rockwell Automation Publication 7000-UM202B-EN-P - June 2014 35
Component Definition and Maintenance Chapter 3
Input Transient Protection The drive provides input transient protection in one of two forms:
• Transient Suppression Network (TSN), or
• Surge arresters
The TSN is optimized for 18 Pulse rectifier designs. Surge arresters are optimized
for AFE and Direct-to-Drive rectifier designs.
Transient Suppression Network (TSN)
The TSN module consists of an assembly of suppressors connected to each of the
three phase input lines and the structure’s ground bus. There are three assemblies
for an 18 Pulse drive.
A transient voltage spike in excess of the semiconductor rating will destroy or
shorten the lifespan of the device. The TSN module suppresses transient
overvoltages on the drive input, and is a standard feature of the drive. The two
basic blocks of the TSN module are the MOV suppressor and the MOV fuse.
MOV Suppressor
The transient suppressors used in the module are heavy-duty metal oxide
varistors (MOVs). Varistors are voltage dependent, nonlinear resistors. They have
symmetrical voltage/current characteristics similar to back-to-back connected
Zener diodes. The varistor has very high resistance below its voltage rating and
appears as an open circuit.
The leakage current through the device would be very small in this region. When
a voltage transient occurs in which the voltage exceeds the ‘knee’ in the curve, the
varistor resistance changes from its high state by several orders of magnitude to a
very low level. The voltage is clamped for a change in current of several orders of
magnitude (
Figure 26 ).