High-resistance ground (HRG) systems have gained popularity in process applications due to their ability to continue operation after a single line-to-ground fault and to limit the escalation of a single line-to-ground fault to a multiphase event. Part I of the companion paper has set the guidelines for designing safe and reliable HRG systems on industrial power systems used with linear loads and the effect of increased line–ground voltage stress on motor loads during fault conditions. However, literature on characteristics and pitfalls of HRG systems when used with nonlinear power converter systems is limited. Part II investigates anomalies of HRG systems used with nonlinear power converters such as neutral voltage shifting, increased line–ground transient fault voltages for both motor and ac source terminals and their voltage compatibility issues, increased neutral grounding resistor power dissipation, susceptibility of ground fault sensing equipment to nuisance tripping, and dc-bus voltage resonance on faults. Part II also investigates HRG zero-sequence fault current characteristics and paths observed under various fault locations in a common ac or a common dc-bus pulsewidth-modulated adjustable speed drive (ASD) system. Part II concludes with a summary of application guidelines when using ASD HRG systems.

Part II: Application Guidelines for High-Resistance Grounding of Low-Voltage Common AC-Bus and Common DC-Bus PWM Drive Systems

The paper describes a selective high resistance grounding (HRG) system for large industrial facility with two in-plant generators in cogeneration mode and two utility transformers. In a 5-step coordination using sensitive ground fault non-directional and directional relays, ground fault throughout the distribution system are cleared with a maximum pickup setting of 6A, time delay of 0.25s. A major consideration for HRG systems is that the stray capacitance charging current of the system should be small (10–15A) to prevent ground fault damage. If the fault is left in place for some duration for a single line-to-ground fault, insulation stresses occur due to higher line to ground voltages on two un-faulted phases. The paper demonstrates that with selective high resistance grounding, both these concerns are mitigated. A selective HRG is a bridge between the classical concepts of HRG and LRG (low-resistance grounding). The modern advancements in ground sensors and protective relaying can selectively clear low levels of ground fault currents, which make selective HRG a reality.

Selective High-Resistance Grounding System for a Cogeneration Facility

The authors have found a mechanism that may account for recent failures of neutral grounding resistors in 26 kV substations owned by Public Service Electric and Gas Company (PSE&G). A neutral grounding resistor can fail due to a local, high-frequency oscillation involving the inherent inductance of the resistor and the transformer phase-to-neutral coupling capacitance. High voltages within the neutral resistor structure can be developed by the cable ring-down transients initiated by a single-line-to-ground fault on a 26 kV feeder circuit. This failure mechanism has not been previously reported in literature.

A possible mechanism for neutral grounding resistor failures

The selection of a grounding configuration and design of a grounding impedance are of vital importance for the stability and functionality of power systems, including industrial and commercial power systems. These key aspects of grounding systems have been the subject of various standards, industrial codes, and recommended practices. In this paper, a review is provided for the methods and procedures developed for selecting a grounding configuration and designing a grounding impedance in industrial and commercial power systems. The reviewed methods and procedures are compared in terms of the ground potential, ground currents (during fault and non-fault conditions), overvoltage during asymmetrical faults, and phase inversion. Several tests are also conducted to draw conclusions and suggest recommendations for the adequate selection of a grounding configuration and design of a grounding impedance. 

Features and Capabilities of Grounding System Designs

Power system safety is heavily related to electrical system grounding, and ground fault current that directly impacts human safety. Protective device characteristics (such as circuit breakers, relays and fuses) are examined and then related to ground fault current levels and trip times. Recommendations are given to improve the traditional grounding concepts. Finally, the design guidelines on safety are summarized, including the transfer voltage problems.

Safety and ground fault protection in electrical systems

Low-voltage (LV) and medium-voltage (MV) industrial power distribution systems below 15 kV are discussed. The method chosen for system neutral grounding is significant to system performance, operation, maintenance, safety, etc. Solidly grounded (SG), low-resistance grounded (LRG), and high-resistance grounded (HRG) systems are discussed and compared. Selection of neutral grounding resistor values and ground fault protection devices are discussed. It is proposed that a modified high-resistance grounding (MHRG) with ground fault tripping may be the better practice in some industrial applications. >

R.L. Henriks

Papers

Selection of system neutral grounding resistor and ground fault protection for industrial power systems