The design of a facility and its electrical equipment should include consideration for future maintenance. In order to remain in good, safe condition, the electrical equipment and facilities must be maintained properly.
Dust and dirt, damaged enclosures and components, corrosion, loose connections, and reduced operating clearances can be the cause of employee injuries.
Some of these conditions can also lead to fire. A thorough, periodic preventive maintenance plan should be established as soon as new facilities and equipment are installed.
Local procedures should be created as soon as possible to cover the maintenance of electrical equipment. Most of this information can be obtained from recognized standards and manufacturer's literature.
Proper operation and maintenance are important to electrical safety because when things do not function as designed or planned, the results may be unexpected.
Many injuries and fatalities have occurred when the unexpected happened. NFPA 70B-1998 is an excellent guide to recommended practices for maintenance of electrical equipment.
It also contains the "why's" and the "wherefore's" of an electrical maintenance program, as well as guidance for maintaining and testing specific types of electrical equipment.
In addition, it contains information in its appendix regarding the suggested frequencies for performance of maintenance and testing. This is a good document to review while facilities are being installed.
SOLID GROUNDING OF POWER SYSTEM BASIC INFORMATION AND TUTORIALS
What is a solidly grounded system?
Solid grounding refers to the connection of the neutral of a generator, power transformer, or grounding transformer directly to the station ground or to the earth. Because of the reactance of the grounded generator or transformer in series with the neutral circuit, a solid ground connection does not provide a zero-impedance neutral circuit.
If the reactance of the system zero-sequence circuit is too great with respect to the system positive-sequence reactance, the objectives sought in grounding, principally freedom from transient overvoltages, may not be achieved.
This is rarely a problem in typical industrial and commercial power systems. The zero-sequence impedance of most generators used in these systems is much lower than the positive-sequence impedance of these generators.
The zero-sequence impedance of a delta-wye transformer will not exceed the transformer's positive sequence impedance. There are, however, conditions under which relatively high zero-sequence impedance may occur.
One of these conditions is a power system fed by several generators and/or transformers in parallel. If the neutral of only one source is grounded, it is possible for the zero-sequence impedance of the grounded source to exceed the effective positive-sequence impedance of the several sources in parallel.
Another such condition may occur where power is distributed to remote facilities by an overhead line without a metallic ground return path. In this case, the return path for ground-fault current is through the earth, and, even though both the neutral of the source and the nonconducting parts at the load may be grounded with well-made electrodes, the ground return path includes the impedance of both of these ground electrodes.
This impedance may be significant. Another significant source of zero sequence impedance is the large line-to-ground spacing of the overhead line.
To ensure the benefits of solid grounding, it is necessary to determine the degree of grounding provided in the system. A good guide in answering this question is the magnitude of ground-fault current as compared to the system threephase fault current.
The higher the ground-fault current in relation to the three-phase fault current the greater the degree of grounding in the system.
Effectively grounded systems will have a line-to-ground short circuit current of at least 60% of the three-phase short-circuit value. In terms of resistance and reactance, effective grounding of a system is accomplished only when R0</=X1 and X0 </= 3X1 and such relationships exist at any point in the system.
The X1 component used in the above relation is the Thevenin equivalent positive-sequence reactance of the complete system including the subtransient reactance of all rotating machines.
Application of surge arresters for grounded-neutral service requires that the system be effectively grounded.
REACTANCE GROUNDING OF POWER SYSTEM BASIC INFORMATION
What is reactance grounding? How reactance grounding is beneficial?
The term reactance grounding describes the case in which a reactor is connected between the system neutral and ground.
Since the ground-fault that may flow in a reactance-grounded system is a function of the neutral reactance, the magnitude of the ground-fault current is often used as a criterion for describing the degree of grounding.
In a reactance-grounded system, the available ground-fault current should be at least 25% and preferably 60% of the threephase fault current to prevent serious transient overvoltages (X0 </=X1).
This is considerably higher than the level of fault current desirable in a resistance-grounded system, and therefore reactance grounding is usually not considered an alternative to resistance grounding.
In most generators, solid grounding, that is, grounding without external impedance, may permit the maximum ground fault current from the generator to exceed the maximum three-phase fault current that the generator can deliver and for which its windings are braced.
Consequently, neutral-grounded generators should be grounded through a low-value reactor that will limit the ground-fault current to a value no greater than the generator three-phase fault current.
In the case of three-phase four-wire systems, the limitation of ground-fault current to 100% of the three-phase fault current is usually practical without interfering with normal four-wire operation.
In practice, reactance grounding is generally used only in this case and to ground substation transformers with similar characteristics.
The term reactance grounding describes the case in which a reactor is connected between the system neutral and ground.
Since the ground-fault that may flow in a reactance-grounded system is a function of the neutral reactance, the magnitude of the ground-fault current is often used as a criterion for describing the degree of grounding.
In a reactance-grounded system, the available ground-fault current should be at least 25% and preferably 60% of the threephase fault current to prevent serious transient overvoltages (X0 </=X1).
This is considerably higher than the level of fault current desirable in a resistance-grounded system, and therefore reactance grounding is usually not considered an alternative to resistance grounding.
In most generators, solid grounding, that is, grounding without external impedance, may permit the maximum ground fault current from the generator to exceed the maximum three-phase fault current that the generator can deliver and for which its windings are braced.
Consequently, neutral-grounded generators should be grounded through a low-value reactor that will limit the ground-fault current to a value no greater than the generator three-phase fault current.
In the case of three-phase four-wire systems, the limitation of ground-fault current to 100% of the three-phase fault current is usually practical without interfering with normal four-wire operation.
In practice, reactance grounding is generally used only in this case and to ground substation transformers with similar characteristics.
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