Abstract [+]
Choosing appropriate ground-fault protection and neutral-grounding resistor (NGR) let-through
current requires careful consideration of several factors including system voltage, ground-fault relay coordination, tripping action, harmonics, and system charging current. In order to facilitate system design and troubleshooting, this paper reviews charging current and sympathetic tripping, protective relay coordination, and the factors that affect NGR selection for alarming and tripping applications. A systematic method is proposed for selecting an NGR, and setting ground-fault protection.
D. Selkirk
May 2008
Abstract [+]
Many of the problems associated with ungrounded and solidly grounded distribution and utilization systems are overcome with resistance grounding. Resistance grounding can limit point-of-fault damage, eliminate transient overvoltages, reduce the flash hazard, limit voltage exposure to personnel, and provide adequate tripping levels for selective ground-fault detection and coordination. Charging current, ground-fault detection, and ground-fault coordination are reviewed. Reasons for monitoring the neutral-grounding resistor (NGR) are presented. Problems associated with NGR monitoring are discussed and monitor design requirements are summarized. These design requirements, and two decades of experience, guided development of two generations of NGR monitors which detect both resistor faults and ground faults.
G.E. Paulson, M.J. Savostianik
July 2003
Abstract [+]
Current flowing to earth has only two paths—it can flow to earth through an earth fault, and it can flow to earth through distributed capacitance. Current flowing to earth through distributed capacitance can cause sympathetic tripping during an earth fault and it can cause nuisance tripping during normal operation. If the earth-fault trip level is high enough to eliminate sympathetic tripping, nuisance tripping due to unbalanced and harmonic capacitive current is usually not a problem. However, if sympathetic tripping is not a concern and earth-fault trip levels are lowered, nuisance tripping can become a problem that worsens with the increased use of adjustable-speed drives. This paper discusses the sources of current flowing to earth that are not the result of an earth fault, and it shows how a digital filter tuned to the fundamental component of earthfault current can provide lower trip levels without nuisance tripping.
M.J. Savostianik
May 2000
Abstract [+]
Rated power is the motor parameter always specified when motors are selected for a belt conveyor—motor slip is usually ignored. This paper shows how the running and starting characteristics of a belt conveyor are influenced by slip. It shows that high-slip motors improve load sharing between directly coupled motors, and it shows that high-slip motors reduce the effect of belt stretch to improve load sharing between belt-coupled drums. The interaction between stretch and slip is illustrated graphically to show the percentage of connected power available to a conveyor without overloading the motor(s) driving the secondary drum. If the power requirement for the conveyor has been determined correctly and if the power available is inadequate, the stretch-to-slip ratio is too high—probably the result of an inadvertent selection of high-efficiency motors with low slip and poor starting characteristics. With these motors, mechanical devices that introduce slip are required if the conveyor is to operate near design capacity. A preferable solution is to avoid the problem by using directly coupled high-slip motors to improve load sharing and increase starting torque. Examples are given of two motors that eliminate the need to introduce slip mechanically.
G.E. Paulson
September 1998
Abstract [+]
Resistance grounding offers many of the advantages of both solidly grounded and ungrounded systems. This paper describes a practical approach to the selection of a grounding resistor let-through current and the operating value of the ground-fault relays. When portable line-power cables are used, ground conductors must be monitored. It is suggested that ground-conductor monitors should be resistance sensitive and that the resistance trip-level should be determined by the allowable ground-fault voltage and the operating value of the ground-fault relays.
G.E. Paulson, J.J. Dudiak
1995