Power

Power
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Senin, 18 April 2011

Cable


6.1 Introduction
Cables for power distribution are mainly characterised by the material of the conductor, voltage grade, type of insulation, type of sheathing, type of armouring etc. Cables are designed to safely carry the current it is required to carry under normal operating conditions as well as abnormal conditions. To carry a current safely means (1) the temperature rise and voltage drop should be within the prescribed limits and (2) the insulation of the cable should withstand the normal operating voltage as well as higher voltages that may arise under abnormal conditions. In some situations, the cable may encounter with severe mechanical stress or highly contaminated environment in addition to the electrical stress. Hence it is necessary to ensure by means of pre-test inspections and various field tests that the cable is capable of withstanding the various types of stresses it may come across during its service life.
6.2 Cable installation plan
Cables are usually laid according to a plan prepared for the purpose. However, after completing the cable laying, it is necessary to prepare a revised plan incorporating the changes made during execution of the work. This plan will help in locating faults and in carrying out tests during the service life of the cable. The cable installation plan should contain the following details.
(1) Location of cables and joints with reference to some fixed landmarks like buildings, boundary stones etc.
(2) Type of cables, cross sectional area, rated voltage and details of cable core.
(3) Length of cables – between joints and ends.
(4) Year and month of laying.
(5) Date of making joints.
(6) Results of measurements and tests carried out on the cable.
While designating the cable the following code shall be used.
6.3 Pre-Commission checks
The following points may be checked as pre-test verification before carrying out the pre-commission tests.
(i) Check for concurrence with the approval issued by the Department of Electrical Inspectorate for
a) the type/designation of cables.
b) the cross sectional area of the conductors.
c) length of cables.
d) voltage grade of cables.
e) direction of power flow.
(ii) Check the cable terminations and cable entry boxes for
a) proper crimping, seating, surface contact etc. of core connections.
b) proper selection of entry boxes to avoid bending.
c) straightness of cable for a minimum length of 45 cms.
d) clamping of cables at 45 cms. from cable termination and subsequent intervals.
(iii) Check the cable gland for
a) proper size and fixing of the gland.
b) size of the earthing conductor.
c) size of earthing clamps.
d) proper surface contact of the earth clamps and for continuity of earth connection.
e) tinning of contact surfaces.
f) perfection of fixing the armour for continuity.
g) double compression glands in hazardous locations.
(iv) Check the cable bends for minimum bending radius as per Table 6.1 and clamping of the cable at both ends of the bend.
(v) Check the cable route for
a) enroute cable clamping.
b) cable dressing in trays and racks.
c) cable identification tags. In the case of UG (Under Ground) cables, the identification tags shall preferably be at intervals of 10 metres minimum.
d) mechanical protection at cable crossings and take off from ground.
(vi) In the case of High Rise Buildings, examine
a) the mechanical protection at each floor crossing.
b) fire prevention barrier in cable ducts at each floor crossing.
(vii) Check for segregation of cables of different voltages.
(viii) Check for segregation of control and power cables.
(ix) Examine the cable alleys, cable trays and racks for
a) segregation of cables
b) cable alley compartmentalisation
c) support in the alleys, trays and racks
d) running of earth continuity strips, adequacy of size of the strip, tap off, joints etc.
(x) Check for the mechanical protection of cables rising from
a) floors
b) trenches
c) ground
(xi) Check for the manufacturer’s identification throughout the length of the cable. Manufacturer’s name or trade mark, the voltage grade and year of manufacture shall be indented, printed or embossed on the cable.
(xii)Check whether separate racks are provided for control and power cables of different voltages in cable trenches of OH (Over Head) trays. Check whether sufficient moving space is provided between racks and side walls of the trench.
(xiii)Check whether the bottom most rack is used for the highest voltage cable and upper racks for lower voltages. Check whether d.c and a.c control cables are segregated.
(xiv)Check whether fire resistant Low Smoke Cables are used in hazardous areas.
(xv) Check whether cable trenches are filled with dry sand in hazardous locations
6.4 Pre-commission tests
6.4.1 Insulation Resistance Test
All new cables shall be tested for insulation resistance before making joints / connections. IR of cables is measured by means of insulation resistance testers. It is preferable to use motorised insulation testers for measuring the IR of cables. The recommended ratings of insulation tester is given below.
Insulation Resistance values to be measured for cables.
In the case of non-screened cables, the IR of each core is measured against all other cores and armour/metal sheath connected to earth. With screened constructions, the IR of each core is measured against all other cores and the metal screen connected to earth. IR of cable is measured after application of test voltage for 60 seconds. The minimum value of the insulation resistance depends on the type of construction of the cable and hence it is necessary to compare the test readings with the factory test results Reasonable variation from the factory test results need not be a cause for concern as the insulation resistance value varies with parameters like length of the cable and temperature. The above variation is more predominant in PVC cables. Once satisfactory results are obtained, cable jointing and termination can be made. Repeat the test after completion of the joints and terminations. Insulation resistance values of cables of various sections of the entire installation shall be measured and recorded in a register for future reference. The ambient temperature at the time of measurement shall also be recorded.
6.4.2 Conductor Resistance Test
The conductor resistance test is carried out for long cables especially where there are joints in the cable. The resistance is measured using a suitable bridge. The conductors at the far end are connected together to form a loop. The connecting bond shall have a cross section atleast that of the conductor.
The measured value of the loop resistance is converted to resistance in ohms per kilometer per conductor using the formula
R = Rl /2L
R is the resistance in ohm per conductor per km
Rl is the measured value of resistance of the cable loop
L is the length of the cable (not loop ) in km
The conductor resistance calculated above may be converted to resistance at 20 deg.C by the formula
The above value of conductor resistance may be compared with the values given in the test certificate of the manufacturer. Wide difference between the two values is an indication of improper joints. Contact resistances are kept minimum by properly clamping or bolting connections.
6.4.3 Measurement of Capacitance
Capacitance of cables is measured for voltages above 11 kV. The measurement is made using a suitable capacitance bridge. In the case of screened cables the capacitance is measured between the conductor and the screen. For unscreened cables, the capacitance is measured between one conductor and others with the metal sheath/armour connected to earth. The measured values are compared with the values given in the test certificate.
6.4.4 High Voltage Test
After making joints and terminations, cables are subjected to High Voltage Test. The condition of the insulation of the cable is evaluated by applying a voltage higher than the rated voltage for a short duration. The cable shall withstand the test voltages given in table 6.2 when applied for a period of 5 minutes.
The test can be conducted by applying dc voltage also. DC test equipments are compact and portable and they require less power.
Test Procedure
The high voltage source is connected to the conductor of the core under test. The cores not under test, screen and armour are connected to the earth terminal, depending on the mode of connection (please see the figures below).
The voltage is now raised slowly, but not so slowly as to cause unnecessary prolongation of the stress near the test voltage. The test voltage is raised to the specific value given in table 6.2 and is maintained for the specified time of 5 minutes. After the specified time, the voltage is suddenly decreased but without sudden interruption to avoid the possibility of switching transients. Insulation resistance values of the cable before and after the HV test shall be measured and compared. There shall not be wide variation in the IR values. Fig. 6.1
Points to note
1. The Cable cores must be discharged on completion of HV test and the cable earthed before it is put into service.
2. For old cables, the test voltage may be limited to 1.5 times the rated voltage. But in no case shall the test voltage be less than the rated voltage.
3. It is not desirable to conduct frequent HV tests on cable installations. The test shall be carried out when the cable is first installed and thereafter only when essential.
4. During the HV test on cables, other electrical equipments such as switches, breakers etc. shall be earthed and adequate clearance maintained from the test equipment and the cable under test to prevent any flash over.
5. In each test, the metallic sheath, screen and armour shall be connected to earth.
6.4.5 Partial discharge and Tan delta measurement
Partial discharge and tan delta values of HV and EHV cables are measured as bench mark references after the cable installation is complete. Partial discharge is measured using a partial discharge measurement instrument and tan delta with a tan delta measuring instrument. The values are compared with the factory test values and recorded for future reference.
6. 5 List of Indian Standards relevant to testing of cables

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