Читать книгу Practical Power Plant Engineering - Zark Bedalov - Страница 81

The electrical equipment must be designed with the insulation withstand capability to operate in the area subjected to lightning strikes and switching surges. Switching surges are not concerned with this plant, i.e. to the voltages 230 kV and lower. The main substation must be properly shielded and protected with appropriately rated lightning arresters (Figure 2.15) placed close to the electrical equipment and grounded by connecting to the transformer enclosure and base and then to the substation grounding grid, using the shortest path possible to ground.

This approach will also protect the transformers from lightning strikes that may be arriving over the overhead line. Similarly, lightning arresters must be installed at all the distribution lines where overhead lines merge with cables or transformers. The transition points: cable to line, etc., are the reflection points for the lightning traveling waves (see Chapter 10 for insulation coordination).

Figure 2.15 Lightning arrester.

The insulation level provided on any piece of apparatus, the transformers in particular, constitute an appreciable part of the cost. The equipment withstand capability is called basic insulation level or simply BIL, for instance 13.8 kV switchgear operating outdoors must be rated for BIL is equal to 110 kVpeak, while the same type of switchgear operating indoors is allowed to be rated 95 kVpeak.

The electrical equipment is, therefore, furnished with lightning arresters, which operate as valves to discharge (spark over) the overvoltage strikes to ground, while a smaller tolerable portion of overvoltage below the equipment BIL level continues to the equipment. The basic insulation level or BIL determines the dielectric strength of the apparatus and is expressed by peak value of the full wave withstand waveform as shown in Figure 2.16.

The voltage withstand capacity of all the equipment in an electrical substation or on an electrical transmission system must be determined based on its operating system voltage as shown in Table 2.4.

Figure 2.16 BIL impulse wave shape.

Table 2.4 System BILs.

System (kV)	BIL (kVpeak)
11	75
13.8	95/110
33	200
66	450
132	650/750
230	900/1050