The present-day electrical power system generally generates, is transmitted and distributed in form of alternating current. The electric power is produced at the power stations which are located in far places, generally away from the consumers. It is delivered to the consumers through a large network of transmission and distribution. At many places in the line of the power system, it may be desirable and necessary to change some characteristics (e.g. voltage, A.C. to D.C, frequency, p.f. etc.) of the electric supply. This is acquired by a suitable apparatus called a substation. For example, the generation voltage (11 kV or 6.6 kV) at the power station is stepped up to high voltage (say 220 kV or 132 kV) for transmission of electric power.
Definition of Substation (Mains Function of Substation):
- Substation is defined as the assembly of apparatus where we change the characteristics of power such as voltage, frequency, power factor, waveform (AC-DC or DC-AC) etc.
- The type of equipment needed in a sub-station will depend upon the service requirement.
Classification or Types of Substation:
(1) Service requirements and
(2) Constructional features
1. Classification of sub-station according to the service requirement:
A sub-station may be called upon to change voltage level or improve power factor or convert a.c. power into d.c. power etc. Based on the service requirement, sub-stations may be classified into:
(i) Transformer substations:
Those sub-stations which change the voltage level of the electric supply are called transformer sub-stations. These sub-stations receive power at some voltage and deliver it at some other voltage. Obviously, the transformer will be the main component in such substations. Most of the sub-stations in the power system are of this type.(ii) Switching substations:
These sub-stations do not change the voltage level i.e. incoming and outgoing lines have the same voltage. However, they simply perform the switching operations of power lines.(iii) Power factor correction substations:
Those sub-stations which improve the power factor of the system are called power factor correction sub-stations. Such sub-stations are generally located at the receiving end of transmission lines. These substations generally use synchronous condensers as the power factor improvement equipment.(iv) Frequency changer substations:
Those sub-stations which change the supply frequency are known as frequency changer sub-stations. Such a frequency change may be required for industrial utilization.(v) Converting substation:
Those sub-stations which change a.c. power into d.c. power is called converting sub-stations. These sub-stations receive a.c. power and convert it into d.c. power with suitable apparatus (e.g. inverter) to supply for such purposes as traction, electroplating, electric welding etc.(vi) Industrial substations:
Those sub-stations which supply power to individual industrial concerns are known as industrial sub-stations.2. Classification of substations according to constructional features:
A sub-station has many components (e.g. circuit breakers, switches, fuses, instruments etc.) which must be housed properly to ensure continuous and reliable service. According to constructional features, the sub-stations are classified as :
(i) Indoor substation
(ii) Outdoor substation
(iii) Underground substation
(iv) Pole-mounted substation
(i) Indoor substations:
- For voltages up to 11 kV, the equipment of the sub-station is installed indoors because of economic considerations.
- However, when the atmosphere is contaminated with impurities, these substations can be erected for voltages up to 66 kV.
(ii) Outdoor substations:
- For voltages beyond 66 kV, equipment is invariably installed outdoors. It is because, for such voltages, the clearances between conductors and the space required for switches, circuit breakers and other equipment becomes so great that it is not economical to install the equipment indoors.
(iii) Underground substations:
- In thickly populated areas, the space available for equipment and building is limited and the cost of land is high. Under such situations, the sub-station is created underground.
Comparison between Outdoor and Indoor Substations:
Features | Outdoor Substation | Indoor Substation |
Location | Installed in open areas. | Installed inside a building or enclosed space. |
Space Requirement | Requires more space for equipment and clearance. | Requires less space due to compact design and enclosure. |
Cost | Lower initial cost due to less building and installation work. | Higher initial cost due to the need for buildings and enclosures. |
Maintenance | Easier to maintain and access equipment. | Maintenance is more difficult due to confined space. |
Exposure to Elements | Exposed to weather conditions like rain, wind, and lightning. | Protected from weather, resulting in fewer environmental effects. |
Safety | Riskier due to exposure to environmental factors. | Safer as equipment is protected from external hazards. |
Use in Urban Areas | Less suitable due to space constraints and noise. | Suitable for urban areas where space is limited. |
Installation Time | Quicker to install due to open space. | Takes more time to construct due to building requirements. |
Risk of Corrosion | High risk of corrosion and damage due to exposure. | Low risk of corrosion as equipment is enclosed. |
Fault Detection | Easier to identify faults visually in open-air setups. | More challenging to detect faults without specialized equipment. |
Voltage Level | Generally used for high and extra-high voltage applications. | Suitable for low to medium voltage applications. |
Clearance | Electrical equipment is individually insulated, allowing for minimal clearance between components. | Overhead lines or busbars are not insulated, necessitating greater clearance for safety. |
(iv) Pole-mounted substation:
- This is an outdoor substation with equipment installed overhead on an H-pole or 4-pole structure.
- It is the cheapest form of substation for voltages not exceeding 11kV (or 33 kV in some cases). Electric power is almost distributed in localities through such substations.
- Figure shows the layout of the pole-mounted substation whereas the transformer and other equipment are mounted on an H-type pole (or 4-pole structure).
- The 11 kV line is connected to the transformer (11 kV / 400 V) through a gang isolator and fuses.
- The lightning arresters are installed on the H.T. side to protect the substation from lightning strikes.
- The transformer steps down the voltage to 400 V, 3-phase, 4-wire supply.
- The voltage between any two lines is 400 V whereas the voltage between any line and neutral is 230 V.
- The oil circuit breaker (O.C.B.) installed on the L.T. side automatically isolates the transformer from the consumers in the event of any fault.
- Pole-mounted substations are generally used for transformer capacities up to 200 kVA.
The following points may be noted about pole-mounted substations :
- There should be a periodical check-up of the dielectric strength of oil in the transformer and O.C.B.
- In case of repair of transformer or O.C.B., both gang isolator and O.C.B. should be shut off.
FAQs related to Substation:
What are the differences between a Power Station and a Substation:
Feature | Power Station | Substation |
Function | Generates electricity by converting various forms of energy (e.g., thermal, hydro, nuclear) into electrical power. | Transforms and distributes electrical power at different voltage levels. |
Location | Typically located far from cities due to space and safety needs. | Located near load centers to distribute electricity to consumers. |
Voltage | Produces electricity at high voltage (typically 11-33 kV). | Steps up or steps down the voltage as needed for transmission or distribution. |
Components | Includes generators, turbines, boilers, and control systems. | Includes transformers, circuit breakers, switches, and busbars. |
Role in Grid | First point of electricity generation in the power grid. | Acts as an intermediary in the transmission and distribution process. |
Size | Large infrastructure requiring significant land and resources. | Smaller infrastructure compared to power stations. |
Energy Source | Uses fuel like coal, natural gas, water, nuclear, or renewable sources like wind or solar. | Does not generate energy but distributes electricity received from power stations. |
Operation Type | Operates continuously to meet demand. | Operates based on demand for electricity distribution and grid stability. |
Environmental Impact | Higher environmental impact due to energy generation processes (e.g., emissions, water usage). | Low environmental impact, as it deals mainly with transmission. |
Which equipment is used in the substation?
1. Power Transformers
- Purpose: Step-up or step-down the voltage levels for efficient power transmission and distribution.
- Types: Step-up transformers, step-down transformers, auto-transformers.
2. Circuit Breakers
- Purpose: Automatically interrupt the electrical circuit to prevent damage during fault conditions (overload, short circuit).
- Types: Air circuit breakers, oil circuit breakers, vacuum circuit breakers, SF6 circuit breakers.
3. Isolators (Disconnectors)
- Purpose: Isolate a part of the substation for maintenance, ensuring that the circuit is de-energized.
- Note: Isolators do not interrupt current; they are operated only when the circuit is open.
4. Busbars
- Purpose: Distribute power between incoming and outgoing circuits in the substation.
- Types: Single bus, double bus, ring bus, and mesh configurations.
5. Surge Arresters (Lightning Arresters)
- Purpose: Protect substation equipment from over-voltage transients caused by lightning or switching surges.
6. Capacitor Banks
- Purpose: Improve power factor and regulate voltage in the substation.
7. Instrument Transformers
- Purpose: Step down high voltages and currents to measurable levels for metering and protection.
- Current Transformers (CTs): Step down current for measurement and protection.
- Potential Transformers (PTs): Step down voltage for measurement and protection.
8. Relays
- Purpose: Provide protection by detecting abnormal conditions (faults) and sending signals to circuit breakers to trip.
9. Control Panels
- Purpose: Monitor and control the operation of substation equipment.
10. Earthing Switch
- Purpose: Ground parts of the circuit for maintenance and protection from high voltage.
11. Batteries
- Purpose: Provide backup DC power for control, protection, and communication systems during outages.
12. Switchgear
- Purpose: Enclose circuit breakers and other switching devices for protection and isolation.
13. HV and LV Cables
- Purpose: Connect various components in the substation, transmitting power at high and low voltage levels.
14. Auxiliary Transformers
- Purpose: Provide power for auxiliary services within the substation (e.g., lighting, cooling systems).
15. Communication Equipment
What is a substation in an electrical power system?
A substation is a facility where characteristics of electrical power, such as voltage, frequency, power factor, or type (AC to DC), are changed to ensure efficient transmission and distribution of electricity.
What are the main functions of a substation?
The main functions of a substation include voltage transformation, power factor correction, AC to DC conversion, and switching operations to ensure safe and efficient electricity distribution.
What are the types of substations based on service requirements?
Substations can be classified based on service requirements into:
- Transformer substations
- Switching substations
- Power factor correction substations
- Frequency changer substations
- Converting substations
- Industrial substations
What are the types of substations based on constructional features?
Based on constructional features, substations are classified into:
- Indoor substations
- Outdoor substations
- Underground substations
- Pole-mounted substations
What is the difference between indoor and outdoor substations?
- Indoor substations: Installed inside buildings, suitable for voltages up to 66 kV, require less space, and are protected from environmental elements.
- Outdoor substations: Installed in open areas, suitable for higher voltages (beyond 66 kV), require more space due to larger equipment and clearances.
What is a transformer substation?
- A transformer substation changes the voltage level of the electric supply. It is the most common type of substation, where the main component is the transformer.
What is the role of a switching substation?
- A switching substation performs switching operations of power lines without changing the voltage level. It ensures power can be directed as needed within the grid.
What are power factor correction substations?
- These substations are used to improve the power factor of the electrical system, typically located at the receiving end of transmission lines, often utilizing synchronous condensers.
What is the function of a frequency changer substation?
- A frequency changer substation alters the frequency of the supply to meet the specific needs of industrial operations or other applications.
What is a pole-mounted substation?
- A pole-mounted substation is an outdoor substation where equipment is installed on poles. It is mainly used for low-voltage distribution (up to 11 kV or 33 kV), making it the most cost-effective solution for local power distribution.
What equipment is commonly found in a substation?
- Common substation equipment includes transformers, circuit breakers, isolators, busbars, lightning arresters, relays, and switchgear for efficient power management and protection.
What are the benefits of underground substations?
- Underground substations are ideal for densely populated areas where space is limited and land costs are high.
- These substations are built below ground to minimize space requirements and environmental impact.
How do converting substations work?
- Converting substations change AC power into DC power, typically for applications like electric traction, electroplating, or welding, using equipment such as rectifiers.