Planned maintenance schedules for various components of the power distribution systems are carefully drawn up by the power distribution utility even before the installation work is completed. During plant shut-downs for overall maintenance and before re-energisation, the sub-transmission and distribution plants are subjected to certain inspection and testing procedures.This also applies to the cable route that has been de-energised for a long period of time. Such planned shut down of the plant to be tested and network reconfiguration ensure continuity of supply to consumers while the testing takes place.
The power distribution utility must formulate such planned outage schemes at different times of the year (depending upon the load demands) for different maintenance periods in such a fashion that consumer supply is least affected.This also involves putting in place a system for handling customer complaints about power supply breakdowns.
Customer Relationship Management System
A trouble call management facility should be provided to attend to the power supply interruptions promptly and to improve the reliability of power supply as well as minimise the down time. It should also attend to fuse off calls promptly as well as the complaints of the customer on quality of supply. A computer based facility provided in the substation/complaint attending centre would
certainly improve this aspect of O&M.We now describe some general maintenance practices for the substation equipment and distribution lines.
General Maintenance Practices
There are two aspects of general maintenance:
Firstly, replacement of the parts that are worn out during the normal operation must be carried out from time to time. Secondly, preventive maintenance should be carried out for detecting deterioration and mal-operation of the system components.In the daily operation of the substation it is the duty of the attendant to inspect the equipment externally and remedy any abnormality that does not require disconnection of the apparatus. During this inspection, a watch is required to be kept for deposits of dust and dirt on the equipment, heating of contacts,joint or some part, low oil level and oil leakages, etc. Checks should also be made to ensure that
• the locks and doors of the switch house are in good condition,
• no leaks have developed in the roof,• the ventilating and heating systems are operating normally,
• the prescribed safety aids are in place and in good order,
• the earthing connections remain unbroken,
• the packing of the cables entering or leaving a cable trench or tunnel within the premises are intact,
• the ventilating louvers are not damaged, and
• the access roads leading to the oil filled apparatus are unobstructed,and will allow the approach of the fire engines in the event of an oil fire during an emergency.
On-line inspection and testing is normally limited to visual, external and physical examination in order to ensure that the plant is in a safe condition. Infra-red detectors must be used periodically for inspection of overhead lines and open terminal, substation bus-bars for hot spots caused by faulty terminals. In addition, live line washing techniques are also available for cleaning overhead lines or open terminal substation insulators.Purified water with a high resistance value is used in a fine spray fitted from well-earthed nozzle. Functional testing and trip schemes require special switching arrangements initially to reconfigure the power system network.
Switchgear site tests during operational maintenance stage vary from utility to utility depending upon the quality of upkeep of the equipment and environmental conditions of the site. These generally involve the following checks and tests:
• General checks include inspection and checking of
− the tightness of terminal connection, piping junctions and bolted joints;
− painting and corrosion protection;
− cleanliness;
− cracking and chipping of bushings;
− foundation bolts; and
− lubrication of contacts and moving parts of the circuit breakers.
• Electrical circuit checks include checking of insulation check;
− dielectric strength of the insulating oil;
− level of the oil;
− quality of SF 6 gas/ insulating medium such as humidity content, filling pressure or density except for sealed apparatus;
− leakage of oil, etc.
Mechanical tests include
− inspection of operating circuits (hydraulic, pneumatic, spring charged) and consumption during operation;
•verification of correct rated operating sequence (recharging, etc).
•Time checks include checking and adjustment of
− track alignment and interlocking mechanism;
− closing and opening times;
− operation and control of auxiliary circuits;
− recharging time of operating mechanism after specified sequence; and
− other specific operations.
Electric tests include
− dielectric tests; and
− testing of the resistance of main circuit.
If the substation is constantly attended, the rounds of switchgear are usually planned for each shift so that all the equipment will be looked at least once a day. Equipment is also inspected immediately after a trip out.Substation switchgear requires regular cleaning in accordance with its design,type of insulation, the degree of pollution of the atmosphere or ambient air,etc. The frequency of cleaning depends upon the type of layout of the apparatus and insulators. However, cleaning must be done during each preventive maintenance activity.
Even though the vacuum switchgear does not require elaborate maintenance like the oil insulated switchgear, it is still necessary to make periodic routine inspection. The absence of ionized gas and carbon during interruption removes the major source of insulation contamination.
Maintenance of Lines
Pre-monsoon inspection of all 33 kV lines should be completed between January and March every year after obtaining due approvals for pre-arranged shut downs for the entire programme.The staff responsible for the pre-monsoon inspection should carry all the necessary equipment such as ropes, petroleum jelly, cotton waste and sufficient O&M materials like insulators, discs, nuts for the pins, binding wire,etc.
In the routine maintenance practices, all the tree clearances are done and all the minor defects like damaged insulators, improper pin binding,loose jumpering and loose stays are rectified during the inspection itself. All the insulators are cleaned, all AB switches are lubricated and defective blades replaced. The defects that may take considerable time for rectification are noted down and attended within the next one week. Examples are insertion of poles, replacement of damaged conductors, replacement of damaged
supports, etc.
Periodical patrolling of 33 kV lines has to be done on a monthly basis. Thepatrolling is also done and suspected defects rectified, whenever the line trips on fault. One of the major precautions to be kept in mind by the maintenance staff is to take the permit to work or line clear to work on distribution lines.
Procedure for Permit to Work (Line Clear)
A line clear or a permit to work (PTW) on any electrical equipment or line is issued by an authorised person to another authorised person. If there are more than one gangs working under the same supervisor, each gang takes sub-line clears from the supervisor who has taken the line clear. In case, the line clear has to be issued for the supervisor, s/he takes self line clear. In this case also, all the precautions that are to be followed in issue and return of line clear are followed.
Line clear books are very important records. Pages in these books are serially numbered and no paper from this book is used for any other purpose. If any page is to be destroyed, the custodian specifically mentions the reasons for doing so. It is attested by his/her dated signature. The line clear books are reviewed periodically by the Competent Authority.Line clear can be issued/received over telephone. It is desirable that the issuer/receiver recognise each other’s voice. The requisition for line clear and the line clear issue messages are repeated by both the parties to ensure that line clears are issued/received on the equipment on which it is intended. A secret code number is followed in such cases.
The power distribution utility must formulate such planned outage schemes at different times of the year (depending upon the load demands) for different maintenance periods in such a fashion that consumer supply is least affected.This also involves putting in place a system for handling customer complaints about power supply breakdowns.
Customer Relationship Management System
A trouble call management facility should be provided to attend to the power supply interruptions promptly and to improve the reliability of power supply as well as minimise the down time. It should also attend to fuse off calls promptly as well as the complaints of the customer on quality of supply. A computer based facility provided in the substation/complaint attending centre would
certainly improve this aspect of O&M.We now describe some general maintenance practices for the substation equipment and distribution lines.
General Maintenance Practices
There are two aspects of general maintenance:
Firstly, replacement of the parts that are worn out during the normal operation must be carried out from time to time. Secondly, preventive maintenance should be carried out for detecting deterioration and mal-operation of the system components.In the daily operation of the substation it is the duty of the attendant to inspect the equipment externally and remedy any abnormality that does not require disconnection of the apparatus. During this inspection, a watch is required to be kept for deposits of dust and dirt on the equipment, heating of contacts,joint or some part, low oil level and oil leakages, etc. Checks should also be made to ensure that
• the locks and doors of the switch house are in good condition,
• no leaks have developed in the roof,• the ventilating and heating systems are operating normally,
• the prescribed safety aids are in place and in good order,
• the earthing connections remain unbroken,
• the packing of the cables entering or leaving a cable trench or tunnel within the premises are intact,
• the ventilating louvers are not damaged, and
• the access roads leading to the oil filled apparatus are unobstructed,and will allow the approach of the fire engines in the event of an oil fire during an emergency.
On-line inspection and testing is normally limited to visual, external and physical examination in order to ensure that the plant is in a safe condition. Infra-red detectors must be used periodically for inspection of overhead lines and open terminal, substation bus-bars for hot spots caused by faulty terminals. In addition, live line washing techniques are also available for cleaning overhead lines or open terminal substation insulators.Purified water with a high resistance value is used in a fine spray fitted from well-earthed nozzle. Functional testing and trip schemes require special switching arrangements initially to reconfigure the power system network.
Switchgear site tests during operational maintenance stage vary from utility to utility depending upon the quality of upkeep of the equipment and environmental conditions of the site. These generally involve the following checks and tests:
• General checks include inspection and checking of
− the tightness of terminal connection, piping junctions and bolted joints;
− painting and corrosion protection;
− cleanliness;
− cracking and chipping of bushings;
− foundation bolts; and
− lubrication of contacts and moving parts of the circuit breakers.
• Electrical circuit checks include checking of insulation check;
− dielectric strength of the insulating oil;
− level of the oil;
− quality of SF 6 gas/ insulating medium such as humidity content, filling pressure or density except for sealed apparatus;
− leakage of oil, etc.
Mechanical tests include
− inspection of operating circuits (hydraulic, pneumatic, spring charged) and consumption during operation;
•verification of correct rated operating sequence (recharging, etc).
•Time checks include checking and adjustment of
− track alignment and interlocking mechanism;
− closing and opening times;
− operation and control of auxiliary circuits;
− recharging time of operating mechanism after specified sequence; and
− other specific operations.
Electric tests include
− dielectric tests; and
− testing of the resistance of main circuit.
If the substation is constantly attended, the rounds of switchgear are usually planned for each shift so that all the equipment will be looked at least once a day. Equipment is also inspected immediately after a trip out.Substation switchgear requires regular cleaning in accordance with its design,type of insulation, the degree of pollution of the atmosphere or ambient air,etc. The frequency of cleaning depends upon the type of layout of the apparatus and insulators. However, cleaning must be done during each preventive maintenance activity.
Even though the vacuum switchgear does not require elaborate maintenance like the oil insulated switchgear, it is still necessary to make periodic routine inspection. The absence of ionized gas and carbon during interruption removes the major source of insulation contamination.
Maintenance of Lines
Pre-monsoon inspection of all 33 kV lines should be completed between January and March every year after obtaining due approvals for pre-arranged shut downs for the entire programme.The staff responsible for the pre-monsoon inspection should carry all the necessary equipment such as ropes, petroleum jelly, cotton waste and sufficient O&M materials like insulators, discs, nuts for the pins, binding wire,etc.
In the routine maintenance practices, all the tree clearances are done and all the minor defects like damaged insulators, improper pin binding,loose jumpering and loose stays are rectified during the inspection itself. All the insulators are cleaned, all AB switches are lubricated and defective blades replaced. The defects that may take considerable time for rectification are noted down and attended within the next one week. Examples are insertion of poles, replacement of damaged conductors, replacement of damaged
supports, etc.
Periodical patrolling of 33 kV lines has to be done on a monthly basis. Thepatrolling is also done and suspected defects rectified, whenever the line trips on fault. One of the major precautions to be kept in mind by the maintenance staff is to take the permit to work or line clear to work on distribution lines.
Procedure for Permit to Work (Line Clear)
A line clear or a permit to work (PTW) on any electrical equipment or line is issued by an authorised person to another authorised person. If there are more than one gangs working under the same supervisor, each gang takes sub-line clears from the supervisor who has taken the line clear. In case, the line clear has to be issued for the supervisor, s/he takes self line clear. In this case also, all the precautions that are to be followed in issue and return of line clear are followed.
Line clear books are very important records. Pages in these books are serially numbered and no paper from this book is used for any other purpose. If any page is to be destroyed, the custodian specifically mentions the reasons for doing so. It is attested by his/her dated signature. The line clear books are reviewed periodically by the Competent Authority.Line clear can be issued/received over telephone. It is desirable that the issuer/receiver recognise each other’s voice. The requisition for line clear and the line clear issue messages are repeated by both the parties to ensure that line clears are issued/received on the equipment on which it is intended. A secret code number is followed in such cases.
Operation and Maintenance of Capacitors
A routine check of the capacitor performance is made by measuring current with the help of Ammeter/Tong tester once in two months and the record is maintained. If any reduction in current /failure of capacitor is noticed, supplier/ manufacturers must be contacted immediately and replacement of capacitor initiated.
The status of the capacitor is determined by the voltage at the highest voltage bus available at the substation. It is subject to the maximum permissible voltage at the bus on which the capacitor bank is connected and the loading factor. The loading factor is the ratio of the total MVA load on the bus at which the capacitor is installed to the MVAR rating of the capacitor. Accordingly, theswitching on/off of the capacitor bank is done as per Table .
The status of the capacitor is determined by the voltage at the highest voltage bus available at the substation. It is subject to the maximum permissible voltage at the bus on which the capacitor bank is connected and the loading factor. The loading factor is the ratio of the total MVA load on the bus at which the capacitor is installed to the MVAR rating of the capacitor. Accordingly, theswitching on/off of the capacitor bank is done as per Table .
 |
| Voltage of Highest Level at the Substation |
The loading factor and the status of capacitor switch are given in Table
 |
| Loading Factor and the Status of Capacitor Switch |
LV bus voltage is controlled by changing transformer taps. Notwithstanding the above, if the voltage at the bus on which capacitor is connected is 1.1 per unit or higher, the capacitor is switched off.
Hot Line Maintenance
Work performed on transmission and distribution lines while they are energized and in service is called hot line maintenance. Hot line tools are all types of tools mounted on insulated poles used to maintain energized high voltage lines and other safety equipment. Insulated disconnect stick,wire-holding stick, auxiliary arm, cross-arm mount, pole mount, wire tong, saddles,flexible line hose and hoist link stick are some of the hot line tools in use.
Work performed on transmission and distribution lines while they are energized and in service is called hot line maintenance. Hot line tools are all types of tools mounted on insulated poles used to maintain energized high voltage lines and other safety equipment. Insulated disconnect stick,wire-holding stick, auxiliary arm, cross-arm mount, pole mount, wire tong, saddles,flexible line hose and hoist link stick are some of the hot line tools in use.
 |
| Hot Line Maintenance |
When working with energized power lines, linemen must use protection to eliminate any contact with the energized line. Some distribution-level voltages can be worked using rubber gloves. The limit of how high a voltage can be worked using rubber gloves varies from company to company according to different safety standards and local laws. YVoltages higher than those (which can be worked using gloves) are worked with special sticks known as hot-line tools, with which power lines can be safely handled from a distance. Linemen must also wear special rubber insulating gear when working with live wires to protect against any accidental contact with the wire. The buckets from which linemen sometimes work are also insulated using rubber.
For high voltage and extra-high voltage transmission lines, specially trained personnel use so-called “live-line” techniques to allow hands-on contact with energized equipment. In this case, the worker is electrically connected to the high voltage line so that he is at the same electrical potential. The lineman wears special conductive clothing which is connected to the live power line, at an instant such that the line and the lineman are at the same potential allowing the lineman to handle the wire safely. Since training for such operations is lengthy, and still presents a danger to personnel, only very important transmission lines are the objects of live-line maintenance practices.
No comments:
Post a Comment