Ground-Fault Protection and VFDs

Product catalog summary

Overview
Ground faults are significant hazards that require mitigation due to their potential to cause severe injuries or fatalities. High-resistance grounding systems provide substantial protection but face challenges in detecting low-level ground faults, particularly when variable frequency drives (VFDs) are involved. These undetected faults can lead to electrical shock and arc-flash hazards, especially in environments like underground mines.

High-Resistance Grounding System Benefits
These systems connect the transformer's neutral point to the ground via a neutral-grounding resistor, which limits ground-fault current, reduces damage, lowers voltage at equipment frames, and minimizes arc flash risks during the initial phase-to-ground fault.

Sensing Ground Faults
Current-sensing ground-fault relays are essential for detecting currents below the neutral-grounding resistor rating. Zero-sequence current transformers are employed to detect unintended current flow, triggering alarms or isolating the faulted section.

Limiting Factors
Detection of low-level ground-fault currents can be limited by factors such as system capacitance, charging current, voltage unbalance, and current transformer limitations. VFDs introduce harmonics and switching frequencies that further complicate detection.

Challenges with VFDs
VFDs complicate ground-fault detection in high-resistance grounded systems due to their built-in protection mechanisms that only trip at higher current levels. Sensitive wideband AC and DC ground-fault protection is necessary for safe VFD operation in these systems.

Harmonic Currents
VFDs generate harmonic currents that can flow through system capacitance to ground, potentially causing ground-fault trips. Solutions include adjusting trip settings or using relays with digital filters to reject harmonic currents.

Detecting Low-Frequency Faults
AC-DC sensitive ground-fault relays, such as the Littelfuse EL731, can detect ground currents across a wide frequency spectrum, offering comprehensive protection in systems with VFDs.

Conclusion
Despite the challenges posed by VFDs for ground-fault detection in high-resistance grounded systems, modern relays can effectively address these issues, enhancing safety and reliability.

Key Definitions
Ground-Fault Current Path: The conductive path from a ground fault point through non-current-carrying conductors, equipment, or the earth to the electrical supply source.
Ground-Fault Protection of Equipment: A system designed to protect equipment from damaging line-to-ground fault currents by disconnecting all ungrounded conductors of the faulted circuit.
Grounding Electrode: A conducting object that establishes a direct connection to the earth.
Grounding Electrode Conductor: A conductor used to connect the system grounded conductor, or the equipment, to a grounding electrode or a point on the grounding electrode system.
Harmonics: Voltage or current with a frequency that is an integer multiple of the fundamental frequency.
Heating: Occurs as electrical current passes through the system, with overcurrent causing rapid heating.
Overcurrent: Any current exceeding the rated current of equipment or the ampacity of a conductor.
Ungrounded: Not connected to ground or a conductive body extending the ground connection.
Voltage: The greatest root-mean-square (rms) difference of potential between any two conductors of the circuit.

Disclaimer Notice: Information is believed to be accurate and reliable, but users should independently evaluate the suitability of each product for their applications. Littelfuse products are not designed for all applications. Full disclaimer available at Littelfuse's website.

Catalog excerpts

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

Open the catalog to page 1

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...

Open the catalog to page 2

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...

Open the catalog to page 3

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...

Open the catalog to page 4

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...

Open the catalog to page 5

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...

Open the catalog to page 6