- Transformers
- Transformer Selection and Sizing
- Ratings of Circuit Breakers
- Ratings of Isolators
- Voltage Transformer
- Current Transformer
- Low Power Current Transformers
- Standard Ratings
- Design & Calculations
- Busbar Size Calculation
- Short Circuit Current Calculation
- Capacitor Bank Size Calculation
- Cable Size Calculation
- Voltage Drop Calculation
- Useful Resources
- Substation Design - Useful Resources
- Substation Design - Discussion
Ratings of Isolators in Substation Design
Isolators are one of the key equipment used in electrical switchyard section of the substation. Isolation are also called as disconnect switch. The main function of an isolator in a substation is to provide a visible break in the circuit during maintenance to provide extra layer of safety. To ensure reliability and safety in the substation, we need to select an isolator of correct rating.
Read this chapter to learn the different ratings or specifications of an isolator and will understand how to select a right isolator for a substation.
Name Plate Data of Isolator
The following table provides the sample name plate data of an isolator for reference −
| Parameter | Value |
|---|---|
| Manufacturer | New Fine Electricals |
| Item | High-Voltage Isolator |
| Type | Double Break Center Post Type |
| Rated Voltage | 36 kV |
| Rated Current | 630 A |
| Rated Short Time Current | 25 kA for 3 sec |
| Rated Short Time Peak Current | 62.5 kA |
| Year of Manufacturing | 2019 |
| Property of | PVVNL |
Ratings of Isolators
In this section, we will learn about different ratings of an isolator, and will use the above table to practically understand the selection a correct rating isolator for a substation application −
Rated Voltage
The rated voltage an isolator is the maximum value of system voltage at which the isolator is designed to operate. The isolator can handle this voltage under both normal operating condition and transient.
In practical substation design, the rated voltage must match the highest operating voltage of the system and not just the nominal voltage. For example, if our substation has a nominal voltage 33 kV, then the rated voltage must be greater than 33 kV, as in above data sheet, it is 36 kV.
Rated Current
Rated current is defined as the maximum amount of current that the isolator can carry continuously without getting overheated.
In practice, an isolator is selected with a rated current equal to or greater than maximum load current of the equipment or feeder to which it is connected. It is also recommended to provide a safety margin of 10% to 20% for future load expansion.
Rated Short-Time Current
The rated short time current of an isolator is defined as the maximum current that the isolator can handle thermally for a certain duration under a short circuit condition.
It is important to understand that an isolator is not designed to interrupt the short circuit current, so it must be designed in a way that it can withstand this current safely until the circuit breaker clears the fault. For example, in name plate, the isolator can withstand a short-circuit current of 25 kA for 3 seconds.
In substation design, an isolator must be selected such that it has a rated short-time current equal to or greater than the short-circuit current level of the system at the point of installation.
Rated Short Time Peak Current
The rated short time peak current is defined as the maximum value of instantaneous current caused by asymmetrical fault currents that the isolator can withstand mechanically. This current rating must match the system conditions, as it allows for withstanding the electromagnetic forces generated during the initial milliseconds of a fault.
The peak current of a system can be calculated by using the following formula −
$$\mathrm{Peak\:Current \:=\: \sqrt{2} \:\times\: Rated\: Short\: Time Current \:\times\: Asymmetry\: Factor}$$
In practice, the value of asymmetry factor lies between 1.6 and 2.5, depending on the X/R ratio of the system.
Selection of Isolator in Substation Design
While selecting an isolator for substation design, we consider the following design factor and their criteria −
| Design Factor | Criteria |
|---|---|
| Rated Voltage | It must be equal to greater than the substation's highest voltage, like 36 kV for 33 kV substation. |
| Rated Current | It must be equal to or greater than the maximum load current of the substation circuit. |
| Short Time Current Rating | It must be equal to or greater than system fault level and generally matched with rating of the circuit breaker. |
| Short Time Peak Current | It must be equal to or greater than peak asymmetrical fault current. |
Example
Let we are designing a substation of 33/11 kV having following specifications −
- Feeder load = 500 A
- Short-circuit level = 20 kA
- Peak asymmetrical fault current = 50 kA (nearly)
Then, we can select an isolator which has −
- Rated voltage = 36 kV
- Rated current 630 A
- Rated short-time current = 25 kA for 3 Sec
- Rated short-time peak current = 62.5 kA
This isolator meets all the technical requirements of our substation with a safe margin.
Conclusion
In this chapter, we covered all about the ratings of isolator and their importance in selecting a right isolator for substation design. We explained all the key ratings and their importance which are specified on name plate and play a critical role in substation design. Apart from this, design engineers must refer applicable standards such as IS9921 or IEC 62271-102 or utility guidelines during final selection of the isolator.