GROUND-FAULT PROTECTION AND VFDs Variable-Frequency Drives Create a Blind Spot to Low-Level Ground Faults in High-Resistance Grounded Systems TECHNICAL PAPER

GROUND-FAULT PROTECTION AND VFDs Overview Death and injuries make ground (earth) faults one of the most essential hazards to mitigate. High-resistance grounding provides significant protection for equipment and personnel. Unfortunately, however, when variable frequency drives (VFDs) are used in resistance-grounded systems, the system may not be able to effectively detect low-level ground faults unless the equipment is adjusted accordingly. Grounding Resistor Power Transformer Secondary or Generator Windings When VFDs are used in high-resistance grounded systems, low-level ground faults can easily...

GROUND-FAULT PROTECTION AND VFDs Limiting Factors Many factors can limit the system’s ability to measure low-level ground-fault currents. These include system capacitance, charging current, voltage unbalance, current transformer limitations, as well as irregularities caused by VFDs (such as harmonics and switching frequencies). System Capacitance All electrical systems have phase-to-ground capacitance, which can be modeled as “lumped” values (see Figure 2). If the capacitances on all three phases in a grounded system are equal, then a zero-sequence current transformer on the three phases together...

GROUND-FAULT PROTECTION AND VFDs Motor Inrush Current to prevent trips from harmonic noise. VFD applications often operate between a 0 Hz to 120 Hz (or more) range. However, most ground-fault relays do not work at low output frequencies (below approximately 20 Hz)—let alone dc. This can make it challenging to get comprehensive protection from a standard ground-fault relay with a VFD on the same system. Motor starting can be problematic because you cannot predict where the switch will close along the ac cycle. Sudden changes in current, like the inrush current of a motor, can cause a momentary...

GROUND-FAULT PROTECTION AND VFDs How to Detect Low-Frequency Faults in Systems with VFDs Other Features Other features of the EL731 include a Ac-dc sensitive ground-fault relays (see Figure 4) measure ground currents as low as 6 mA, with some able to monitor across a wide frequency spectrum (including 0 Hz for dc applications).  2 x 16-character OLED display  current and temperature metering  menu-navigation push buttons Littelfuse model EL731 microprocessor-based ac-dc sensitive ground-fault relay (also referred to as an earthleakage relay) is for use with up to two sensitive current transformers...

GROUND-FAULT PROTECTION AND VFDs Heating: Heating occurs in every part of the system as the electrical current passes through the system. If the overcurrent is large enough, this will almost instantaneously occur. The energy in such overcurrents is measured in ampere-squared seconds (I2t). An overcurrent of 10,000 A that lasts for 0.01 s has a thermal energy of 1,000,000 I2t. A current of only 7,500 A can melt a #8 AWG copper wire in 0.1 s and within 8 ms (one-half electrical cycle). A current of 6,500 A can raise the temperature of an AWG THHN thermoplastic-insulated copper wire from its operating...

GROUND-FAULT PROTECTION AND VFDS

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