APPLICATION CONSIDERATIONS FOR HIGH …

1 APPLICATION CONSIDERATIONS FOR high RESISTANCEGROUND RETROFITS IN PULP AND PAPER MILLSR obert BeltzMember, IEEEC utler-Hammer3900 Kennesaw 75 , GA 30144 Ian PeacockCutler-HammerRR3 Box 910 FWinthrop, ME 04364 William Vilcheck, Member, IEEEC utler-Hammer130 Commonwealth DriveWarrendale, PA 15086 Abstract - Safety and reliability of Low Voltage-HighResistance Grounding schemes have made them anexcellent choice for APPLICATION in the pulp & paper andother process industries. As facilities shift toward thistechnology for low voltage substations for improvedreliability, retrofitting existing grounded systems requiresseveral critical APPLICATION CONSIDERATIONS . This paper willbriefly review different grounding methods and theirtheory of operation. Then, examples of actualinstallations will be presented that will identify potentialmisapplications of high resistance ground retrofits,discuss what typical ground voltages and currents toexpect, and present important CONSIDERATIONS for reliableand safe installation of HRG Terms high resistance Grounding, Retrofits, ground Fault Detection, Grounding Transformers, Paper MillsI.

2 INTRODUCTIONP ower systems engineers have developed several methodsof effective grounding of industrial power distributions systemsover the years. Historically, the method of system groundingselected for various electrical system settings, industrial,commercial, etc., dates back to the early part of this centurywhen only two methods were considered: solid grounded andungrounded. Solid grounding with its advantage of high faultlevels to drive protective devices into tripping had equallysignificant disadvantages such as dangers posed by arcs inhazardous areas. Also, the issue of service continuity ofcritical loads pointed away from this grounding method. Theperception that ungrounded systems provide servicecontinuity, at least through the first ground fault, stronglysuggested ungrounded systems. In more recent times wellaccepted, if not misapplied grounding techniques utilizingresistance or reactance, have provided the power systemsengineer other alternatives.

3 These techniques as well assolidly grounded and ungrounded systems will be Principles of GroundingA grounding system is isolated from other groundingsystems by delta windings in three-phase systems. It onlytakes one delta winding to accomplish isolation; not bothprimary and secondary windings. Beginning at the maintransformer secondary, there are four separate groundingsystems illustrated in Fig. 1. System 1 includes all of the 480volt system including the source generator through all theprimary delta windings of the loads. The wye ungroundedmotor and wye-wye solidly grounded transformer secondary isalso a part of the 480 volt system. Any grounding problem inthe secondary of transformer T1 will affect the 480 voltsystem. In contrast, any grounding problem in groundingsystems 2, 3 or 4 will not effect its respective primarygrounding system due to its primary delta Grounding systemsWhen the type of grounding to be selected is beingaddressed within the design stage of an electrical powersystem, there are two key questions which must beconsidered:1) Are there any line-to-neutral loads?

4 2) How important is service continuity for this electricalsystem?The answers to these questions strongly influence the typeof grounding selected. Line-to-neutral loads suggest solidgrounding; high continuity of service requirements suggest anungrounded or something approaching an Types of System Grounding1)Solidly grounded systems are commonly found inindustrial/commercial power systems today. In many cases,solid grounding (NEC-Article 250-5) is mandated. In others, itis selected based on economics. The 480/277 V system inFig. 2a is solidly grounded to permit 277 V (line-to-neutral)loads, such as line-to-neutral loads to be applied, the neutral point ofthe wye-connected source must be solidly grounded for thesystem to function properly and safely. If the system is notsolidly grounded, the neutral point of the system would "float"with respect to ground as a function of load subjecting theline-to-neutral loads to voltage unbalances and 120/208 V wye system in Fig.

5 2b must be solidlygrounded to comply with the 1999 NEC Article 250-20 (allsystems with line-to- ground voltages of 150 V or less and forthose systems with line-to-neutral loads). It is selectedprimarily where a lot of 120 V (line-to-neutral) loads 240/120 V 3-phase delta system shown in Fig. 2c,typical of commercial power systems, must also be solidlygrounded to comply with the , the lowest line-to-groundvoltage is less than 150 2. Examples of solidly grounded )The ungrounded circuit historically has been selectedfor those systems where service continuity is of primaryconcern. See Fig. 3. for examples. The perception is thatungrounded systems have higher service continuity. This isbased on the argument that the ground fault current is smalland that negligible burning or heating will occur if the fault isnot cleared.

6 Therefore line-to- ground faults can be left on thesystem until it is convenient to find and clear them. Thisperception has some validity if one limits the criterion to bolted or hard faults. However, in industrial electricalsystems, the vast majority of all faults start as low level arcingground faults. When arcing ground faults are considered, thefollowing conditions surface but are seldom addressed. Multiple ground faults Transient overvoltages Resonant conditionsMultiple ground faults can and do occur on ungroundedsystems. While a ground fault on one phase of an ungroundedsystem does not cause an outage, the longer the ground isallowed to remain the greater is the likelihood of a secondground occurring on another phase because the unfaultedphases have line-to-line voltage impressed on their line-to- ground insulation.

7 The insulation is overstressed by as muchas 73 percent. Also, there is an accelerated degradation of theinsulation system due to the collective overvoltages impingedupon it, through successive ground -faults over a period ofseveral 3. Examples of ungrounded overvoltages due to restriking or intermittentground faults can and do develop substantial overvoltages onungrounded electrical systems with respect to ground . Therehave been many documented cases within industry wheremultiple equipment failures ( ) over an entire 480 Vsystem have occurred while trying to find and locate a groundfault. Measured line-to- ground voltages of 1200 V or higher inthese instances have been reported. In all instances, thecause has been traced to a low-level intermittent arcingground fault on an ungrounded mechanism explaining how this occurs is bestexplained in conjunction with Fig.

8 4. Under worst caseconditions, there is an energy exchange between the systeminductance and the shunt capacitance to ground resulting insignificant voltage escalation with respect to 4. Transient overvoltages from a restriking ground ) Low resistance grounding has beenselected for large electrical systems wherethere is a high investment in capitalequipment or prolonged loss of service ofequipment has a significant economicimpact. A resistor is connected from theTHEORETICALACTUAL system neutral point to ground andgenerally sized to permit only 200 A to 1200A of ground fault current to flow (Fig. 5).4) Enough current must flow such thatprotective devices can detect the faultedcircuit and trip it off-line but not so muchcurrent as to create major damage at thefault point. Damage to motors and otherrotating equipment is also limited.

9 Since thegrounding impedance is in the form ofresistance, any transient overvoltages arequickly damped out and the whole transientovervoltage phenomena is no longerapplicable. This method is employed on2400V and 4160V 5. resistance grounded ) high resistance grounding is almost identical to lowresistance grounding except that the ground fault currentmagnitude is typically limited to 10 A or less. high resistancegrounding, or HRG, applications in paper mills will bediscussed in detail in the next Special ConsiderationsThere are some safety and operational aspects that shouldbe addressed. One of the real hazards with an ungroundedsystem is the occurrence of a second ground fault. Althoughnothing happens after a single ground fault, the secondground fault acts like a phase-to-phase fault. Therefore it isimportant to remove ground faults from ungrounded systemsas soon as possible.

10 This is often difficult. A ground faultdetection scheme provides a systematic procedure forlocating ground area that should be addressed is personnel people are of the opinion that an ungrounded system issafer. This opinion says that since contact with a single phasedoes not complete a circuit, one should not get shocked. Thisis not the case because capacitive coupling to ground ispresent. In Fig. 6., a sustained ground fault occurs on a 480volt wye connected (could be delta) ungrounded the system is capacitively coupled to ground through arelatively high impedance, the unfaulted phase voltage isdisplaced above ground potential. The system will remain inthis position until the fault is cleared, or another phase breaksdown to form a double line-to- ground (480 V)/-j800 = A(.6) COS 30 + (.6) COS 30 = AFig. 6. ground faults on ungrounded systemsAnother area of consideration is continuity of service.