1.4 Ratings
1.4.1 General
The purpose of ratings is to correctly specify GIS equipment based on electric system topology and characteristics while reducing the variety of technical possibilities and guiding manufacturers. These ratings provide standardized solutions recognized across the industry and reduce cost. The main rating parameters are voltage, insulation level, frequency, current, short time and peak withstand current, duration of short circuit and auxiliary voltages, and frequencies.
In high‐voltage switchgear, the rating structures are defined for devices such as circuit breakers, and disconnect switches, grounding (earthing) switches, and connecting conductors/bus. In general, they are covered in IEEE C37.100.1 or in IEC 62271‐1 for switchgear products.
For assemblies of high‐voltage switchgear like GIS, the standards are used to satisfy design criteria and applications in the field. The design of the GIS must consider the very high cost for developments and manufacturing of different types of pressurized metal enclosures. For this reason, some designs of GIS are grouped to cover multiple voltage ratings. One technical criterion of the equipment, for example, rated voltages of 110, 123, 138, and 145 kV, is covered by the same class of GIS with the same enclosure. Once you reach higher voltages of 230, 345, 500 kV and up, you have GIS in separate enclosures.
Within this range of voltage classification, only different gas densities of SF6 differentiate between the different voltage levels. In terms of current ratings, the difference between 2000, 2500, and 3000 A might only be a different number of contact fingers or different wall thicknesses of conductors.
1.4.2 Rated Maximum Voltage
The high voltage (HV) levels in standards start at ratings above 52 kV in both IEC and IEEE. Below these voltage levels, the equipment is classified as medium voltage (MV). The typical GIS high‐voltage ratings can be grouped into four design classes, even if the split may vary somewhat. The lower high‐voltage ratings are in the range from 52 to 72.5 kV as the first level range. The second level range includes 100, 123, 145 kV, and in some cases the 170 kV rating. The third level range covers 245 and 300 kV. The fourth level range includes the voltages 362 and 420 kV. The 345 kV voltage level is considered as 362 kV and is no longer recommended by standards today. The third and fourth level have been historically developed in North America and Europe, typically as 245 and 420 kV in Europe and 300 and 362 kV in North America. The reason behind this is the availability of technical solutions like insulators at the time when the new voltage levels were established. An overview of the voltage ratings is given in Table 1.5.
Only two voltage ratings left in the IEEE and IEC standards, which have different options for power frequency switching and lightning impulse values, are 245 and 362 kV. 550 kV rated voltage offers two insulation levels for the rated power frequency withstand voltage.
1.4.3 Rated Insulation Level
The ratings for insulation levels are derived from the network to which the GIS is connected. Network conditions, like lightning strokes into overhead lines, their local probability, and their expected strength, are indicators for the overvoltage that may occur. In the case of cable networks, the length of cables and their related overvoltage during switching operations will influence this rating.
Rated insulation levels are key parameters for the design of GIS and do have a direct impact on the enclosure diameter and, with this, a high‐cost impact in development and manufacturing cost. Each rated voltage in IEC and IEEE has the choice of two or even more insulation levels. In GIS, the choice is usually made in favor of the highest requirement for the GIS.
As shown in Table 1.5, in most cases the listed rated power frequency withstand voltage, the rated switching impulse withstand voltage, and the rated lightning impulse withstand voltage for the related rated maximum voltage is the highest value from IEC and IEEE standards. Only the rated maximum voltage classifications of 245 and 362 kV have the choice of two voltage levels. The reason behind these choices is that, in North America, many such GIS are in operation from the past, while the rated insulation levels of today’s GIS offer higher values.
Table 1.5 Rated voltages of IEEE and IEC
| Rated max voltage U m | Rated power frequency withstand voltage | Rated switching impulse withstand voltage | Rated lightning impulse withstand voltage (BIL) | ||
|---|---|---|---|---|---|
| IEC | IEEE | kV rms | kV rms | kV peak | kV peak |
| × | × | 72.5 | 140 | — | 325 |
| × | × | 100 | 185 | — | 450 |
| × | × | 123 | 230 | — | 550 |
| × | × | 145 | 275 | — | 650 |
| × | × | 170 | 325 | — | 730 |
| × | 245 | 425 | — | 900 | |
| × | × | 245 | 460 | — | 1050 |
| × | × | 300 | 460 | 850 | 1050 |
| × | 362 | 500 | 850 | 1050 | |
| × | × | 362 | 520 | 950 | 1175 |
| × | × | 420 |
650
|
