Substation receives the energy transmitted at high voltage from the generating stations and reduces the voltage to a value appropriate for local distribution and also provides facilities for switching. Substations are mainly for switching and converting. In switching substations different connections between various transmission lines are made. Whereas converting substations converts ac into dc or vice versa or converts the frequency from higher to lower or vice versa.
Functions of substation
- Provides safety in case of fault i.e. safety devices are installed to disconnect equipment or circuit in the event of a fault.
- The voltage on the outgoing distribution feeders can be regulated at a substation.
- A synchronous condenser installed at the end of the transmission line improves the Power factor and makes measurements to check the operation of the various parts of the power system.
Classification of substation
Substations are classified on a different basis:
Classification of substations on The Basis of the Nature of Duties
- Step-Up or primary substations: These substations are usually associated with generating stations. The generated voltage is low(3.3,6.6,11 or 33 kv) and is stepped up to the primary transmission voltage.
- Primary Grid Substations: It is located at suitable load centers along the primary transmission lines. In such substations, the primary transmission voltage is stepped down to different suitable secondary voltages. The secondary transmission lines are carried over to the secondary substations situated at the load centers where the voltage is further stepped down to sub-transmission or primary distribution voltages.
- Step-Down or Distribution Substations: It is located at the load centers where the sub-transmission/primary distribution voltage is stepped down to the secondary distribution voltage(415/240 V). These substations feed the consumers through distribution networks and service lines.
Classification of substations on The Basis of service Rendered
- Transformer Substations: Transformers are installed on such substations to transform the power from one voltage level to another level as per needs.
- Switching substations: These substations are used for switching the operation of power lines without transforming the voltage.
- Converting substations: These substations are used for converting ac to dc or vice versa or converting frequency from higher to lower or vice versa.
Classification of substations on The Basis of operating voltage
- High voltage substations(HV substations): voltage between 11 Kv and 66 Kv.
- Extra High Voltage Substations(EHV substations): voltage between 132 Kv and 400 Kv.
- Ultra High Voltage Substations (UHV substations): Voltage above 400 Kv.
Classification of Substations on The Basis of Importance
- Grid substations: Bulk power is transmitted from one point to another point in the grid using this substation.
- Town substations: These substations step down the voltages at 33/11 Kv for further distribution in towns and any failure in such substations results in the failure of supply for the whole of the town.
Classification of Substations on The Basis of Design
- Indoor Type substations: The apparatus is installed within the substation building. Such substations are usually for a voltage up to 11KV but can be erected for the 33KV and 66 KV.
- Outdoor substations: These substations are further divided into:
A. Pole-mounted substations: These types of substations are erected for the distribution of power in localities. Single stout pole or H-pole and 4-Pole structures with suitable platforms are employed for transformers of capacity up to 25 KVA,125 KVA, and above 125 KVA up to 250 KVA.
B. Foundation-mounted substations: This type of substation is usually employed for voltages of 33,000 volts and above.
The apparatus is installed within the substation building. These types of substations are usually for a voltage up to 11,000 V but can be erected for 33,000 V and 66,000 volts when the surrounding atmosphere is contaminated with impurities such as metal corroding gases and fumes, conductive dust, etc. The switchgear on the supply or primary side consists of an oil circuit breaker only. The high voltage supply is given to the primary of the transformer through the circuit breaker, and various feeders emerge from the Bus Bar arrangement.
The panel for each feeder consists of an isolator switch and circuit breaker and provides measuring instruments. For the protection of panel power, Protective Relays is used. For the protection of oil-filled transformers with conservator tanks Buchholz’s relay is used.
There are various auxiliaries in indoor substations:
- Storage batteries are used for the operation of the protective gear and switch operating solenoids & emergency lightning in substations in case of failure of supply.
- firefighting equipment such as water buckets, fire extinguishers, etc.
The various compartments of indoor substations are:
- Control compartment.
- Indicating and metering instruments and protective device compartments.
- circuit breaker and operating mechanism compartment.
- main bus-bar compartment.
- current transformer and cable sealing box compartment.
According to construction indoor distribution transformer substations and high-voltage switchboards are further subdivided into the following categories:
- Substations of The Integrally Built Type: In this type of substation the apparatus is installed on-site and the structure of the site is constructed using concrete or bricks.
- Substations of The composite Built-Up Type: The assemblies and parts are factory or workshop prefabricated and are assembled on-site within a substation switchgear room.
- Unit Type Factory Fabricated substations and metal clad switchboards: In this type of substation the parts are made in electrical workshops and are shipped to the site of installations fully pre-assembled.And when the installations of substations and switchyard are completed only the connection to the incoming and outgoing power circuits is required to be made. Metal clad is designed with withdrawable trucks and is divided into several compartments.
Outdoor substations are very widely used in power systems. Outdoor substations are divided into further two types:
- Pole-mounted substations and
- Foundation-mounted substations.
- Pole-mounted substations: These are the cheapest, simple, and smaller substations and are used for mounting distribution transformers of a capacity of up to 250 KVA.Ht distribution lines are used for supporting the structure of outdoor substation equipment. Triple pole mechanically operated (TPMO) switches are used for switching “on” and “off” of ht transmission line.HT fuse unit is installed for protection of ht side. To control lt side iron-clad low-tension switches of suitable capacity with fuses are installed. To protect the transformer from the surge lightning arresters are installed over the ht line. Substations are earthed at two or more places.
Transformers of capacity up to 125 KVA are mounted on double pole structure and for transformers of capacity above 125 KVA but not exceeding 250 KVA 4-pole structures are used.
2. Foundation Mounted substations: These substations are built entirely in the open and in such substations, all the equipment is assembled into one unit usually enclosed by a fence from the point of view of safety. Substations for primary and secondary transmission and for secondary distribution(above 250 KVA) are foundation-mounted outdoor types. The equipment required for such substations is heavy.
Advantages of outdoor substations over indoor substations
- Extension of Installation is easier.
- Fault location is easier as the equipment is within view.
- Lesser time is required for the erection of outdoor substations.
- A smaller amount of building materials is required.
- cost of switchgear required is comparatively low.
- Repairing work is easy.
Disadvantages of outdoor substations over indoor substations
- More space is required for the substation.
- All the maintenance of the apparatus is to be performed in the open air during all kinds of weather.
- Protection devices are required to be installed for protection against lightning surges.
- The length of control cables required is more.