Standard Ratings of Substation Equipment

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Read this chapter to get a good understanding of the standard ratings of major equipment used in MV (Medium-Voltage) or EHV (Extra-High Voltage) substations. It is extremely important to select a proper equipment based on standard ratings to design a reliable MV or EHV substation. In India, it is a standard practice to follow guidelines from the Indian Standards (IS) and the International Electrotechnical Commission (IEC).

This chapter will provide a practical overview of standard ratings of major electrical equipment used in MV or EHV substations. Electrical design engineers can refer this chapter as a guide to make informed design decision for selecting substation equipment.

In this chapter, we will cover the standard ratings of the following substation equipment −

• Current Transformer (CT)
• Potential Transformer (PT) or Capacitive Voltage Transformer (CVT)
• High Rupturing Capacity (HRC) Fuse
• Distribution / Power Transformer
• Switchgear

Standard Ratings of Current Transformers

Current Transformer, commonly known as CT, is an instrument transformer used to reduce high primary currents to a safe low value, usually 1 A or 5 A. CTs are used in substations for metering, protection, and control applications. Selection of correct CT rating ensures accurate measurement and reliable relay operation.

The following table highlights the standard ratings of CTs used in MV and EHV substations in India −

For example, a substation has 220 kV system voltage and line current of 2000 A, then we can use a CT for metering and protection with standard ratings of 2000/1 A, 30 VA, 0.2s (metering), and 5P20 (protection).

Standard Ratings of Potential Transformers

Potential Transformer, commonly called as PT or VT, is another special type of transformer used in electrical substations for metering and protection applications. PT converts the very high voltage of system to a standard low value, usually 110 V phase-to-phase or 63.5 V phase-to-neutral.

Note: In medium voltage substations, an inductive PT is used, while in high voltage substations, a capacitive voltage transformer is used.

The standard ratings of key technical specifications of a potential transformer are given in the following table −

For example, in a 400 kV substation, we can use a 400 kV/110 V, 200 VA CVT of class 0.2 accuracy.

Standard Ratings of HRC Fuse

HRC (High Rupturing Capacity) fuses are most widely used types of fuses in low and medium voltage substations or electrical systems for protecting transformers. But they are not employed in high-voltage or extra-high voltage substations, where circuit breakers are essentially required.

The following table provides standard ratings of key technical specifications of HRC fuses −

Note − In EHV substations, the fault levels are too high and essentially require circuit breakers with protective relays, hence these substations do not use HRC fuses.

Standard Ratings of Distribution and Power Transformers

Distribution transformers are mainly used in LV and MV systems to reduce high voltages to a low value suitable to supply load. The distribution transformers mainly reduce 11 kV or 33 kV voltages to 440 V voltage.

On the other hand, power transformers are larger transformers that can handle huge amount of electric power at very high voltages usually from 66 kV to 765 kV. The power transformers are mainly used in grid substations or generating stations.

The following table provides standard ratings of major technical specifications of distribution and power transformers −

Standard Ratings of Switchgear

Switchgear is a group of equipment that includes circuit breakers, isolators, and ring main units. The table given below provides key standard ratings of switchgears used in medium voltage and EHV substations −

Conclusion

In this chapter, we covered the details of all the major equipment of MV and EHV substations and their standard ratings. It is recommended that the electrical design engineers refer to IS/IEC standards, DISCOM or power company specifications, and future load growth and fault levels to design a reliable electrical substation.