Voltage systems

Figure 12.3 Heating effect caused by an unbalanced voltage system...

Table 13.2 Standard insulation (impulse) Levels for Series 1 range I voltage systems (1 kV < l/, < 245 kV)...

Switchgear and controlgear assemblies 13/345 Table 13.3 Standard insulation (impulse) levels for range II voltage systems (V , > 245 kV)... 

We illustrate a typical powerhouse generation and transmission system layout in Figure 13.21, and reproduce in Table 13.10 the typical fault levels of different transmission and distribution networks in practice for different voltage systems. 

Note For CTs this multiplying factor has been specified as 2.5 for all voltage systems, as in lEC 60044-1. See also Section 15.7 for metering and protection current transformers. 

The test voltage must be as close to a sine wave as practicable and frequency as noted in column 3 of Tables 14.1 and 14.2, for series II and Table 13.2 for series I voltage systems, and applied for one minute as follows ... 

Table 14.3 For series I voltage systems. Dielectric test voltages for control and auxiliary circuits and also LT power circuits...

Table 14.7 Insulation resistance for different voltage systems...

Tappings are generally not necessary, as a transformer is designed for a particular voltage system. If and when such a need arises (as in a control transformer (Section 15.4.5)) they can be provided on the primary side of the transformer. 

For lower voltage systems, say. 2.5 to lU kV, measurement of dielectric loss factor tan 5. along similar lines, to those recommended... 

This is more pronounced on high-voltage and extra-high-voltage systems (66 kV and above) when ... 

Yet to be defined by lEC 60099-4, but generally for low-voltage systems. Not often in use for power system applications. 

These techniques have been successfully implemented in interrupting devices as noted in Section 19.1.2. being commercially produced by various manufacturers for different voltage systems and applications. 

Unlike other mediums, the dielectric strength of a vacuum increases with a gap. but only mtirginally, which is the limiting factor in producing such breakers beyond 36 kV. These breakers are therefore used only for medium-voltage systems (2.4-36 kV). Some manufacturers have attempted to produce them up to 66 kV but they have not shown the desired results so far. The application of these breakers therefore continues to be up to 36 kV only. 

Since a very small gap in vacuum can withstand a very high voltage, a larger gap than required w ill not increase its dielectric strength. This is the limiting factor for a VCB to exceed a certain voltage system, presently 36 kV. 

We use HT for all voltage systems above l.l kV unless a comparative reference is necessary. 

When we refer to a transmission system, we classify the different maximum voltage systems as follows ... 

For higher voltage. systems, refer to the relevant Standards noted in the table of Standards provided at the end of the chapter. 

Consider a bus duct having a rated current of 4000 A and an unbalanced current in the middle phase of 4400 A. Determine the size of the reactor to achieve a balanced voltage system. 

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