Driven by the benefits of resistance-grounding including concerns over arc-flash hazards, many plants that use variable frequency drives (VFDs) have installed high-resistance grounding (HRG) on the power line feeds. Some of these plants have found out the hard way that ground fault protection built into a VFD probably doesn’t work with HRG.
Variable frequency drives are used extensively in heavy industry to control rotating machinery such as compressors, pumps, conveyors, mixers and HVAC equipment. Many, if not most, of these VFDs have an integral ground-fault protection circuit. Lately, there have been reports of damaged drives because of ground faults, particularly in those operating at lower voltages (below 600 VAC).
We first became aware of this problem at a large petroleum refinery in Alberta, Canada. The refinery’s plant engineers didn’t realize the limitations of built-in ground fault protection in the drive systems. If high-resistance grounding (HRG) is installed on the power system, a VFD’s built-in ground fault protection might not work at all.
Origins of the problem
Simply put, a ground fault occurs when electricity travels through ground, instead of the intended path back to its source. More than 80% of electrical failures in equipment are ground faults caused by worn insulation, conductive dust or moisture. Deteriorated insulation on wires and cables cause 90% of these events. In cases where a person accidentally becomes a part of the ground circuit, current as low as 75 mA can send that person’s heart into ventricular fibrillation.
HRG can limit the available ground-fault current throughout a plant. It’s been used in downtime-averse process plants and explosive environments for many years, but its use is growing in other plants because of concerns about the hazards associated with arc flash. HRG limits phase-to-ground fault current and greatly reduces the risk of having an arc flash. Because HRG limits the fault current, it’s an important safety factor in limiting the energy available to a ground fault and preventing a ground fault from escalating to an arcing fault. Many plant engineers overseeing VFDs might not be aware that the drive’s ground-fault protection is incompatible with an HRG system. Many VFDs were designed for use with a solid ground instead of an HRG system. The ground-fault protection built into those VFDs should work with a solid earth ground on the feeder transformer neutral.
This isn’t the case when those VFDs are used with HRG systems. Often, a VFD’s built-in ground fault protection is set to trip at 50% of full load. However, when the VFD and its motor are connected to a system using HRG, ground fault current will never reach that 50% setpoint.
A reliable solution
There’s an easily implemented fix for this problem that can prevent the large expense associated with replacing a damaged VFD and motor. The solution is adding an appropriate ground fault relay (GFR) system, typically a microprocessor-based device that can be used with both HRG and solidly grounded systems. This type of GFR can do more than detect fault current. It can reveal leakage currents as they develop so ground faults are detected before they become critical. In fact, when properly applied, these GFRs act as preventive maintenance devices, alerting operators to a problem before equipment is damaged severely. This could save thousands of dollars in downtime, replacement parts and liability lawsuits. Relative to these costs, the price of a GFR system is negligible.
Figure 1. In this typical GFR and current transformer connection diagram, GF1, GF2 and GF3 are potential ground fault locations that must be detected by the GFR system.
To sense fault current, the GFR is used with a zero-sequence ground fault current transformer (CT). In Figure 1, T1 steps the line voltage down to match the GFR operating voltage. When a ground fault occurs on the CT’s load side, for example, at any of the points labeled GF1, GF2 or GF3, the GFR analyzes the current the CT senses; if the current suggests a ground fault, the GFR operates its output contacts and lights a trip LED. The output contacts can be connected to an alarm system, such as a PLC, or can be connected to stop the VFD or trip an upstream circuit-interrupting device. The front-panel reset button or the remote reset can unlatch the GFR trip after the fault has been corrected. Auto-reset also is selectable.