Electrical

# What’s Inside a Substations (Grid)

## Overview:

• What is substation
• Why we use substation
• What is the working and components of substation?
• How many types of substation are there?
• What is the main equipment of substation and what are there working?

## What are substations?

As we know that electricity is generated at 11kv in generating station and we cannot supply that to our homes and the generating stations are also 50km,100km or 200 km away. It should also be kept in mind that we supply 220v to our homes and in 3-phase supply we have 415v approx. and for that we need a transformer to stepdown the 11kv to our required voltage that is usable in home and the place where transformer is present is known as substation.

## Why we use substations

### Voltage Step-up and Step-down

As discussed above that the power is generated in 11kv. After that, the electricity is step-up to 220kv or 500kv for the transmission in the transmission lines. This transmission is done in a substation. So, the most important part of a substation is to step-up and step-down the voltage.

### Substations used for metering

The power plant like Jamshoro Power station and Karachi Nuclear Power Plant (KANUPP) and several others generate the supply and supply the electricity to the national and power grid.

The generating station belongs to a private company and the transmission lines belong to the government; so, the national grids and the power grids measure the amount of power drawn from the generating stations.

Suppose a grid station has a total load of 1000kw. The 750-kW load belongs to the city and the 250kw is connected in the village, the total load is 1000kw. But if the consumer in the city increases and the load is increased from 750kw to 850kw then the total load will become 1100kw but the station can support only 1000 kilowatt and the breaker of the station will get open. to prevent this scenario the electricity of the village is cut-off and the electricity is given to the city so the electricity demand increases and the load balancing is done by the substation in the area

which means that the substations plays an important role in the balancing of the load as well.

### Safety and power quality

if the generating station is 200 km away from the place where we need to supply electricity we cannot directly supply the electricity from the generating station to the area of supply because if the maintenance occurred in the system then it would be very difficult for us to perform the maintenance and to identify the part where the fault occurred would be very difficult and hectic task for us. so to avoid this scenario we use a substation between the 200 km long transmission line for maintenance and improve the power factor of the applied voltage as well for that we use capacitor Bank in the substation to improve the power factor and this substation will make us able to perform the maintenance of the transmission line is safe and sound condition. By using multiple substations at different distances, we can identify the fault easily and we can also perform the maintenance where necessary. The substation makes us able to identify the fault in the transmission lines easily. Where the capacitor Bank is used where we need power quality correction and mostly the capacitor power bank is installed near the substation where the load is near.

So far, we have seen the use of substations that why we use substations now let’s move to the electrical substation equipment that is used in an electrical substation.

## Electrical Substations Equipment’s: –

### Transformer

The heart of a substation is a transformer we need to protect the Transformer our objective is to protect it from any type of transmission surges and overloading. All the equipment’s in the substation are used for the safety and protection of the transformer so that the transformer is working in a safe and sound condition.

And the function of a transformer is used to step up or step down transmission voltage and current. If the station is a Step Up station, the Transformer will increase the voltage, and the Step Up Transformer will be used to increase the voltage from 11kv t 220kv. And if the station is a step-down station, then the Transformer will decrease the voltage, and the step-down Transformer will be used to decrease the voltage from 132kv to 33kv.

### Lightning arrester

The Lightning arresters are installed on both sides of the transformer. If in case lightning strikes the transmission lines, then the voltage of the transmission line will increase tremendously and the high voltage Surge from the lightning or the generating station in case of high power voltage the Transformer would be damaged. So to avoid any damage, we use lightning arresters on both sides of the transformer. so The purpose of a lightning arrester is to ground the high voltage coming from the transmission lines suppose that the 132KV station has 300k v due to the lightning or generation end fault then the lightning arrester will protect the transformer from the higher voltage and the higher voltage will get grounded. the voltage also rises when we perform the switching in the substation the voltage surges are grounded by the Lightning arrester installed in the substation and the Transformer stays in safe and sound condition

### Circuit Breaker

The circuit breaker provides protection to the Transformer and the transmission lines whenever the current overload, load overload or earth fault happen breaker gets open for the protection of the transformer. In most substations, we use SF6 breakers. the circuit breaker is also installed on both side of the transformer.

### Current transformer (CT)

The current transformer is used for the measurement of current coming into the substation from the generating station. the current transformers are installed in series with the line.

### Isolators

The isolator is an offload device. let’s suppose we wish to perform the maintenance of the transformer so for that reason we need to cut off the transformer from the transmission lines. the person that is the going to the maintenance of the transformer need surety that the transmission line is off by breaker we cannot identify that whether the generating station is cutoff or not so for that we, first of all, cut off the generating transmission lines from the Transformer using a no-load device that is an isolator. there is a gap between the terminals of the isolator so that the voltage wouldn’t jump from one end to the other and causing the spark or voltage arc. the breaker doesn’t have any free space between them so in case of a faulty breaker voltage can jump from one side to the other and could be dangerous for the electrician to work upon so by opening the isolator there will be no chance for potential availability.

Because the terminal of the isolator has enough gap. the breaker is opened first before opening the isolator is known as no/offload device and the breaker is known as on load device.

### Capacitive voltage transformer

Capacitive voltage Transformer is used instead of the potential transformer as the current transformer is used for measuring of current similarly capacitive voltage Transformer is used for measuring voltage. In the substation below 132 Kv, we use potential Transformer PT and in the case of substation greater than 132 kV we use a capacitor potential transformer to measure the voltage. there are also other components in a substation like input line and output line and the ground cable which provide protection to the substations if any high voltage lightning strikes then the ground cable ground the extra voltage.

now let us see how many types of substations are available.

## Types of Substations: –

### classification based on voltage level

• extra High Voltage substations
• High Voltage substations
• medium voltage substations
• low voltage substations
• HVDC substations

substations classified based on location

• outdoor substations
• indoor substations

• underground substations

### substations based on configuration

• air-insulated substation
• sf6 gas insulated substation (GIS)

The substation we see in our daily life are air insulated substation gas substation are very rare in Pakistan.

### Based on application

• step-up substation
• step down substation

The step-up substation is installed near generating station to step up the voltage for transmission and step-down substations are used near load to step down the voltage that is for household applications and to power equipment’s. The step-down substation is also known as distribution substation.