Case Studies > Capturing Softstarter Operation
The PQPro with PV II software is the ideal tool for capturing detailed power system information. In this case study a Softstarter was used to start a 5500Hp induction motor on a relatively weak power system. In addition to the Softstarter a Capacitor bank was connected for power factor compensation.

A PQPro was used to collect the data so that the utility could confirm the effect of this equipment on their system. Measurements were done on the 14.4kV distribution line. The PQPro was configured for continuous waveform storage so that a complete analysis of the data could be done after the test (no trigger levels needed to be set as every waveform for every cycle and RMS data with 200 millisecond resolution is recorded).

Figure 1: Voltage and Current during Motor Softstart operation

The data recorded shows clearly the effect of the Capacitor bank during all phases of the operation. When the Capacitor bank was connected there was a high frequency oscillatory current transient. Until the Softstarter was connected the Capacitor bank was the main load on the system resulting in a power factor of 0.04 Lead. The capacitor bank supplied 7.5MVar. The maximum Var requirement of the motor and Softstarter during starting was 18 MVar which dropped to 6 MVar while running under light load conditions. During starting the Capacitor bank was under compensating and while running lightly loaded was over compensating. The Capacitor bank was comprised of stages that could be disconnected and was designed so that at full load the Capacitor bank and motor would be running at a power factor of 0.95 Lag.

Figure 2: Capacitor Bank Energization

The operation of the Softstarter is also well documented. When the Softstarter is first turned on the system power factor jumps from 0.04 Lead to 0.07 Lag within a few cycles. During the time that the Softstarter was engaged (about 20 seconds) the average power factor was about 0.25 Lag. When the Softstarter was bypassed operating power factor of the combined Capacitor bank and motor under light load was about 0.60 Lead.

Figure 3: Softstarter On
Figure 4: System Power Factor
Figure 5: Low Frequency Current Oscillations Near End of Softstarter Operation


The utility found that the load did not meet its connection requirement for Rapid Voltage Change (RVC). The maximum RVC measured was 8.9% while the connection requirement is a maximum RVC of 6% with no more than 4 per day.

It was also noted that near the end of the Softstarter operation there were low frequency current oscillations. While not considered to be objectionable by the utility it is something to be aware of and may be indicative of potential resonance issues.