Ground fault detectors provide economic efficiency by improving operational availability.
by Mervin Savostianik
July 3, 2018

A 12-pulse input VFD is fed by a transformer with two secondary windings. An 18-pulse input VFD is fed by a transformer with three secondary windings. These configurations result in phase-shifted outputs, allowing for more pulses to be created. In theory, the increase in total pulses reduce the total harmonic distortion (THD) from input to output. In practice, however, some harmonics remain due to high-speed output switching.

To accurately monitor VFDs while online and offline, all input secondary transformers and the output filter to the motor must be monitored with an IMD. However, only one IMD may operate on a system at any given time. Multiple IMDs on the same system may see each other as a ground fault due to their connections to ground and their internal resistances.

The solution was found in one device that could handle all these challenges with the required functionality, reliability and affordability. One ground fault detector was chosen to monitor the entire system, including the transformer secondary windings, the VFD and the motor. The ground fault detector has the ability to detect AC and DC faults while remaining immune to harmonics, and has the ability to coordinate with other IMDs in the same circuit.

One feature allows internal separation between IMDs in the monitored system. When the system is energized, only one IMD remains active to monitor the entire circuit while the others remain on standby.

When the system is de-energized, three IMDs are required for the 12-pulse input (one for each transformer winding and one for the motor) and four IMDs are required for the 18-pulse input systems. The increase in IMDs is due to the presence of the diodes in the rectifier circuit, which do not allow for the flow of current when the system is offline.

Industrial communication and building automation networks are used more frequently in many industries, including wastewater treatment. Relay outputs and two-way fieldbus communication are available for connection to a programmable logic controller (PLC) or other controller.

Additionally, a built-in web server is available to read device values and configure settings in real-time. In this application, maintenance personnel use the web server to get the most up-to-date information on the status of their electrical systems.

The improved operational availability and economic efficiencies experienced at the prominent wastewater treatment facility is a significant demonstration of the versatility and functionality of deploying powerful ground fault monitoring technology in an ungrounded system.

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