Electric Substations — How do they work?
The challenges associated with construction, operation, and maintenance of the power grid are often complicated. Many of those challenges are overcome at the facility end which, at first glance, often look like a chaotic and dangerous mess of wires and equipment, but actually serves a number of essential roles in electrical grid, is the SUBSTATION.
Electrical substations have an utmost importance in electric distribution facility (see video explanation here), converts AC voltages from one level to another level or change nature of supply voltage i.e., from AC to DC or vice versa. The general layout of a substation consists of conductors that run along the entire substation. To avoid shutting down the entire substation, we need switches that can isolate equipment, transfer load, and control the flow of electricity along the bus. Switching in a substation is a carefully controlled procedure with specially designed equipment to handle high voltages for the protection of equipment.
Types of Substation
The substations can be categorized as:
On the Basis of Operation:
A substation can be used to transform voltage levels or for rectification or to improve power factor. Based on service requirement, substations can be classified as:
Transformer Substations
Substation used to step up or stepdown voltage level of AC power system for power distribution is commonly termed as transformer substation. Power substations are usually located near generating stations to increase the generated voltage for transmission of electric power over the long distances. Distribution substations stepdown the voltage to a lower value according to consumer requirement. They are located near the center of load to feed the power to consumers.
Converting Substations
These substations are used in DC transmission system to convert three phase alternating current to direct current and vice versa by deploying converters, harmonic filters, and synchronous condensers at sending and receiving end of transmission system.
Power Factor Substations
To compensate the power losses during the transmission of electric power, synchronous condenser is used as a power factor correction device. Substations which deploy the use of capacitor banks or synchronous condensers are known as power factor correction substations.
Click here to learn more about The Application of Synchronous Condenser
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On the Basis of Construction Features:
Indoor Substation
These substations are divided into different compartments for controlling and metering of devices. Equipment are installed within the building of substation to avoid the exposure of transmission line with hazardous chemicals.
Outdoor Substation
As the name implies, these substations have equipment located out. Outdoor substation require large area clearance between the live conductors. These substations are further classified as:
- Pole Mounted Substations (PMS)–are erected for mounting power distribution transformer in the localities. The pole mounted substation shall be located in non- hazardous overhead obstructions free environment.
- Foundation Mounted Substation / Surface Mounted Substation –mount the transformers on ground surface having capacity of 33,000 volts or above.
Comparison Table of Indoor and Outdoor Substation:
Layout of a Substation:
Components of a Substation
Transformer
- How to select type of transformer?
- What should be the size of transformer?
- Why should you choose a transformer with minimum no load losses?
Switchgear
- Monitoring equipment
- Protection Devices
Click here to read the detail of each component of “Electric Substation”.
Calculation for Feeder Size and Overcurrent Protection
The allowable ampacity of feeder conductor size should not be less than 125% of the continuous load along with the non-continuous load. Article 310 of NFPA 70 NEC 2017 explains the ampacity ratings, mechanical strength, uses and general requirements for the conductor. Table 310.15(B) shows size of conductor based on ambient temperature up to 2000 Volts. For THHW insulated copper conductor at 750 C, required size is 4/0 kcmil for load amperage of 230A.
Table 310.15 (B) Conductor size based on Ampacities of Insulated Conductors
Article 240 of NFPA 70 covers the basic requirements for overcurrent protection devices operated at system rated voltage below and above 1000 volts. The overcurrent protective device should be capable to allow 125% of the continuous and non-continuous load as discussed in Article 215.3.
The steps to calculate feeder size and rating of OCR device can be summarized as:
With the advancement in automation system for electrical substation, an extensive consideration should be given while selecting the equipments to provide high reliability to power systems.
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About The Author
Abdur Rehman is a professional electrical engineer with more than eight years of experience working with equipment from 208V to 115kV in both the Utility and Industrial & Commercial space. He has a particular focus on Power Systems Protection & Engineering Studies.
Abdur Rehman is the CEO and co-founder of allumiax.com and creator of GeneralPAC by AllumiaX. He has been actively involved in various roles in the IEEE Seattle Section, IEEE PES Seattle, IEEE Region 6, and IEEE MGA.
