Creepage Distance for Insulators in Substation

Creepage Distance for Insulators:

The creepage distance for insulators is the shortest distance along the insulator surface between the metal parts at each end of the insulator. Creepage distance can also be refer as leakage distance for insulators.
Insulators in substation are provided to avoid any leakage current from live electrical conductors to flow to the earth through supports. The atmospheric dust sticks to the insulator surface forming a conducting layer. The leakage current flows from the live conductor to the earth through such surface layers. The leakage properties (creepage properties) of an insulator s in substation are characterized by the length of the leakage path. While designing the insulator sheds, the leakage distance for insulators requirement should be satisfied. The requirement of the leakage distance or creepage distance for insulators depends on the

• Rated phase to ground voltage
• Degree of atmospheric pollution

If the surface of the insulator is clean, smooth and well gazed the dust particles tend to fall down. When an a.c voltage is applied to the apparatus, the dust particles have a tendency to align to the direction of electrical field lines on the surface of the insulator. If the surface of the insulator is rough and moist this alignment will be faster. The continuous application of voltage causes slow alignment of these particles. To prevent a continuous conducting track, the insulator should have sufficient ceepage distance or leakage distance.

During power frequency withstand test of an unclean insulator, the flash-over occurs along the dirty surface of the insulator. In case of internal gas filled or oil filled apparatus, the internal surface should also be free from moisture and dust. Otherwise internal flash over can occur along the surface by tracking. Some of the typical creepage distance for insulators in substation provided based on the level of pollution are tabulated below

S No.	Degree of Pollution	Recommended Creepage Distance for Insulators
1	Clean areas	16 mm/kV
2	Moderately polluted areas	20 mm/kV
3	Industrial areas	22 mm/kV
4	Heavily polluted areas	25 mm/kV

Underground Cable Fault Identification Methods

If a fault occurs in the underground cable, it is essential that the type of the fault and location of the fault should be determined as quickly and accurately as possible. Accuracy is important in order to avoid excessive trenching work. The type of fault which is most likely to occur is single conductor to ground fault. In multi-core cables, the fault current will likely give rise to excessive heating at the fault causing further damage to the insulation and extending the fault to remaining conductors. Open circuit faults may occur occasionally which be usually at cable joints. 

Cable fault type identification:

 Prior to the location of the fault on the power system it is important to determine the type of fault so as to make a better choice of the method to be used for fault location

• Isolate the faulty cable and test each core of the cable for earth fault. One terminal of the insulation tester is earthed and each conductor of the cable is in turn touched with other terminals. If the insulation resistance tester indicates zero resistance during any measurement, conductor to earth fault for the particular conductor is confirmed
• Then check the insulation resistance between the conductors. In the case it is a short circuit fault, the insulation resistance tester will indicate zero resistance
• After the above step, short and earth the three conductors of the cable at one end. Check the resistance between the conductors and earth and between individual conductors (at the other end). This procedure is carried out to check the open circuit faults
• In case in order to test any other faults. the insulation test of the individual cores with sheath or armour and between the cores is essential. The test should also be done by reversing the polarity of the insulation resistance tester. In the case of any difference in the readings, the presence of moisture in the cable insulation is confirmed. The moisture in the cable forms a voltage cell between the lead sheath and conductor because of the difference in the conductivity of these metals and the impregnating compound forms an organic acid when water enters 

Cable fault location Identification:

After the fault type identification, suitable fault location method should be employed to pinpoint the location of the fault. Some of the fault identification methods generally employed are:

• Murray loop test method
• Fall of potential test
• dc charge and discharge method
• Induction test
• Impulse wave echo test
• Time domain reflectometry test