In this article, we will discuss an electrical substation overview, including types of substations, AIS & GIS substations, HVDC, and HVAC substations.
What is a Substation?
The electrical power generation from sources such as Hydro, Thermal, Nuclear, Renewable, and other generating stations has to be evacuated to load centers. The generation voltage is limited to 11/13 kV due to the limitation of the rotating machinery.
The generating station voltage needs to increase to a higher voltage to transmit power to far-located load centers. Again, the voltage is lowered to lower voltages at the load centers for onward transmission and distribution. The transmission of power at a higher level reduces the line loss because of ohmic loss(I2R).
There is a need to control the load centers for power dispatch from powerhouses to ultimate consumers. For this, a number of Transformation and switching stations are created. These stations are generally known as substations.
Thus, the Sub-Stations are fitted with the following equipment.
- Switching devices (breakers, isolators, switches, and contactors.)
- Control and measuring instruments
- Indicating, regulating, and protective devices
Need of Substation
Substations are needed in between long primary transmission lines. The main purposes of the substations are the following.
- To avoid switching surges.
- For easy segregation of faulty zones.
- For enabling effective protection of the system in the network.
- To tap and provide different lines extended to different load centers without any step-down facility at the switching stations.
Functions of Substation
Transform- A substation can transform voltage from one voltage level to another.
Dispatching- Establishment of economic load distribution and load shedding to maintain synchronism.
Control -Controlling the exchange of energy along with Voltage Control: reducing the reactive power flow by compensation of reactive power or by tap-changing.
Measurement -Measuring the voltage, current, and other parameters with the help of CT & PT/ CVT for metering purposes.
Protection-Protection of transmission system along with fault analysis.
Communication-Speech and Data transmission via Power Line Carrier/ OPGW for network monitoring, control, and protection.
Operation Criteria
- Ease of operation and control
- Ease of testing the electrical installation
- Safety
- Flexibility
- Reliability
Types of Substations
Below are detailed explanations of the classification of substations based on their functions.
- According to the service
- According to design
- According to Mounting
- According to Operating Voltage
Now, we will discuss the types of substations in detail.
In accordance with the service
- Transformer Sub-station(Step-UP and Step Down)-Depending on the voltage level, it can be a Transmission Substation (Above 33 kV) and a Distribution sub-station (Below 33 kV).
- Industrial Sub-Station– Bulk supply or industrial substation is generally a distribution substation, but they are dedicated to one consumer only.
- Switching Sub-station -The substations used for switching the power line without disturbing the voltage are known as the switching substations.
- Converting Sub-station -In such types of substations, AC power is converted into DC power or vice versa, or it can convert the high frequency to lower frequency or vice versa.
- Frequency Changing Sub-station -This substation is also known as a Back-to-Back substation, where two different system frequencies are synchronized in the HVDC application.
- Mobile Sub-Station– For big construction purposes, this substation fulfills the temporary power requirement during construction work.
Types of Substation According to the design
- Indoor Type: The substations constructed under a roof are called indoor-type substations. Generally, 11 kV and sometimes 33 kV substations are of this type.
- Outdoor Type: Outdoor-type substations are constructed in the open air. Nearly all 132kV, 220kV, and 400kV substations are outdoor-type substations. However, nowadays, special GIS (gas-insulated substations) are constructed for extra-high-voltage systems generally situated under the roof.
Types of Substations According to Mounting
Pole Mounted Substation
- Pole-mounted substations are the type of distribution substation. It is constructed on two poles, four poles, and even more pole structures, depending on the power-delivering capacity of the substation.
- In this type of substation, fuse-protected distribution transformer-250 KVA is mounted on poles along with electrical isolator switches.
- All the equipment is outdoor and mounted on the supporting structures of high-tension distribution lines.
Foundation Mounted Substation
The heavy equipment can not be mounted on the poles. For heavy equipment, a foundation-mounted substation is used. In the foundation-mounted substation, all the equipment is mounted on the foundation, and then the substation is embedded by the fence for safety purposes.
Types of Substations by Operating Voltage
- HV Substations – Operating voltages between 11 kV and 66 kV.
- Extra High Voltage Substations – Operating voltages between 132 kV and 400 kV.
- Ultra High Voltage – Operating voltage is more than 400 kV
Comparison between AIS & GIS
GIS is a Gas-insulated substation, and AIS is an Air-insulated substation.
Installation
Installing a GIS substation is faster than installing its AIS. GIS systems are compact and have less weight than AIS.
Operation
GIS technology is more accessible to care for and monitor regularly as it offers front instead of rear access.
Arc flashes are rare in gas-insulated substations because all the switching elements are thoroughly insulated. The connections parts of GIS are fully insulated; therefore, there is almost no chance of touching the live parts with the earth.
Maintenance
GIS switchyards require significantly less maintenance.
Clearances
With a GIS system, the clearance needed for phase-to-ground or phase-to-phase for all equipment is much less than that of an AIS. The GIS only needs one-tenth of the clearance compared to the clearance of AIS.
Environmentally Protected System
GIS is a good choice for hilly areas or desert locations, as it can be enclosed in a building that is environmentally protected from harsh conditions.
HVDC & HVAC
- Alternating current (AC) is the driving force in industries and utilities, but for long transmission lines (more than 400 miles), HVAC transmission is more expensive than direct current (HVDC). Because of the frequency, AC transmission line control is more complicated.
- DC transmission does not have these limitations, which has led to the building of long HVDC transmission lines such as the Talcher –Kolar 500KV HVDC line.