0093-9994 (c) 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TIA.2014.2363074, IEEE Transactions on Industry Applications PART II: APPLICATION GUIDELINES FOR HIGH RESISTANCE GROUNDING OF LOW VOLTAGE COMMON AC BUS & COMMON DC BUS PWM DRIVE SYSTEMS Gary Skibinski Member, IEEE Rockwell Automation Mequon, WI 53092 Zhijun Liu Member, IEEE Rockwell Automation Mequon,WI 53092 Robert VanLieshout Member, IEEE Rockwell Automation Mequon, WI 53092 Mark Weaver Ben Byman Member, IEEE Sr. Member, IEEE Rockwell Automation Weyerhaeuser Paper Mequon, WI 53092 Longview, WA 98632 Abstract – 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 ability to limit escalation of a single line-to-ground fault into a multi-phase event. Part I of the companion paper set 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 non-linear power converter systems is limited. Part II investigates anomalies of HRG systems used with non-linear 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 Common AC or Common DC bus Pulse Width Modulated (PWM) Adjustable Speed Drive (ASD) system. Part II concludes with a summary of application guidelines when using ASD HRG systems. Index Terms – High Resistance Ground (HRG), Ground Fault Indicators (GFI), Pulse Width Modulation (PWM), Adjustable Speed Drive (ASD) I. INTRODUCTION The trend toward integrating HRG systems into industrial power systems has a long history and it’s evolution over the past 50 years was reviewed in [1]. Advantages of HRG systems are continuity of service, low ground fault current, low frame contact voltage, and reduced equipment damage. Another main HRG advantage is elimination of arc flash hazard on a single line-to-ground fault. HRG design guidelines for connection to sinewave power sources with linear loads was summarized in [1] based on research literature and HRG implementation to existing Standards / Codes given. HRG disadvantages are a minimum 173% of rated VLN line-neutral voltage stress with possible resonant over-voltages to 600% VLN, with a poor RNG neutral resistor selection. Part I investigated maximum allowable terminal voltage stress and maximum fault removal time under sustained ground fault conditions. Analysis showed low voltage motors can tolerate initial over-voltages without immediate breakdown. However, if sustained VLN over-voltage > 3x rated exists, a corona failure mechanism may reduce slot liner Fig.1 HRG RNG damping resistance factor on linear 480 Vrms power system Diode or AFE AC line (1) DC bus (2) Inverter # 1 Inverter # 2 COMMON DC BUS ASD out (3) Motor #1 stator (4) DC (+) DC (-) M M Motor #2 Vs RNG X_input Sarc LINE- GROUND FAULT (a) Common DC bus system with 2 ASDs ; Motor #1 with ground fault (b) Simplified zero sequence circuit of (a) with Motor #1 stator fault Fig.2 HRG damping RNG value less effective on non-linear ASD system insulation life to hours-days for small motors and days-weeks for large motors. If sustained VLN over-voltage < 3x rated exists, standard Voltage Endurance vs. Life long term dielectric aging mechanism of slot insulation is the limiting factor, which allows much more time for coordinated fault removal. Line-to-ground fault studies historically focused on a sinewave HRG system with linear motor loads [2-4]. In Fig.1, a ground fault is represented by switch (Sarc) closing when restrike voltage levels are reached. Energy stored in system Vs RNG PE ground Xco_b X_input AFE So Xco_in AFE 3.2 kHz control DC bus (2) S1_a Sn S1_b X_output_a X_output_b Inverter # 1 2 kHz control Inverter # n 4 kHz control X_output_n X Xco_n Arc switch Sarc Ico_1a LINE- GROUND FAULT ASD out (3) Motor stator (4) AC line (1)