DOI : https://doi.org/10.5281/zenodo.20326564
- Open Access
- Authors : S.Sudha Rani, Dr. M. Nalini Devi, E. Deepika
- Paper ID : IJERTV15IS051634
- Volume & Issue : Volume 15, Issue 05 , May – 2026
- Published (First Online): 21-05-2026
- ISSN (Online) : 2278-0181
- Publisher Name : IJERT
- License:
This work is licensed under a Creative Commons Attribution 4.0 International License
A Comprehensive Study on Design Considerations of 33/11KV Substation
S. Sudha Rani
Assistant Professor EEE, MGIT Hyderabad, India
Dr. M. Nalini Devi
Assistant Professor EEE, MGIT Hyderabad, India
E. Deepika
Student EEE, MGIT Hyderabad, India
Abstract – A substation is a critical component of an electrical power system that facilitates the transformation, control, and distribution of electrical energy. It serves as a node in the power grid, enabling the transfer of electricity between different voltage levels, ensuring efcient and safe delivery to consumers. They are designed with considerations for load growth, fault tolerance, and environmental factors, making them essential for modern power systems.
A substation receives electric power from generating station via incoming transmission line and delivers electrical power via outgoing lines. The main function of substation is to receive elec-trical energy from generating station and transmit at high voltage from the generating station, reduced to a value appropriate for local distribution and provide facilities for switching. Basically, substations consist of power transformer, circuit breakers, isola-tors, relays, earthling, switches, current transformers, potential transformers, synchronous condenser/capacitor banks, etc. The voltage conversion will be obtained by use of power transformer which is considered as a heart of substations.
The design process of a sub-station begins with very elemental work of selection of site and estimation of requirements which includes capital and material. Choosing the right equipment is essential for the efcient and safe operation of the substation. It is also needed to keep in mind, the civil aspects of a sub-station design.
Our project is mainly concerned with the Design considera-tions of 33/11 kV substation the various components present in substation and a special focus on the design considerations of different equipments.
Index Terms33/11 kV Substation, Power Transformer, Cir-cuit Breaker, Protective Relays, Voltage Transformation, Electri-cal Power Distribution.
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Introduction
The generating voltage is generally 11KV or higher, but in some cases, it may be as high as 6.6 kV or 33 kV. It is eco-nomical to generate voltages at lower voltages simultaneously, and it is economical to transmit power at higher voltages.
Bulk electric power is generated at power plants. This is transmitted to power receiving substations through transmis-sion lines. These receiving power substations then distribute it to end-users through distribution lines. The combination of this transmission and distribution network is known as the electric power grid.
Electrical power transmission is done at high voltage, in a range of 33 kV to 765 kV, to minimize transmission losses.
Then this voltage is stepped down and distributed to end-users by using step-down transformers. These transformers reduce the voltage from high voltage to 415 V/220 V.
A well-designed substation ensures reliable power supply, operational safety, and cost-effectiveness while complying with regulatory standards and environmental requirements.
33/11KV Substation forms an important link between the transmission network and distribution network. It has a vital inuence on the reliability service. Substation is constructed as near as possible to the load Centre. Site selection of substation includes various factors such as
-
Availability of land and sufcient land
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Communication facility
-
Atmospheric pollution
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Availability of essential amenities to the staff
-
Future expansion/upgrade of substation
A. Single Line Diagram of 33/11KV Substation
A single line diagram also called the one-line diagram is a symbolic or graphical representation of a power system. It has a diagrammatic representation of all the equipment and con-nections. Standardized schematic symbols represent electrical elements such as circuit breakers, transformers, bus bars, and conductors, making them easy to read and understand. In a single line diagram, instead of representing each of the three phases with separate lines, a single conductor represents all three phases. A single line diagram makes it easy to understand an electrical system, particularly in the case of complicated systems in substations.
Single line diagrams are invaluable tools in the electrical power industry, providing a clear and concise representation of complex power systems. They simplify design, analysis, and maintenance processes, ensuring safety and efciency.
-
-
SUBSTATION EQUIPMENTS
Equipment selection is a crucial aspect of designing of 33/11KV substation. It involves choosing the right equipment that meets the technical, safety, and reliability requirements of the substation. Substation is the place where there us an assembly of various electrical apparatus to perform several transformations and switching action
Fig. 1. Single Line Diagram of 33/11KV Substation
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Busbars and Bus coupler
Busbars are critical components that serve as a common connection point for multiple circuits, facilitating the distri-bution and control of electrical power. Busbars are metallic conductors (typically copper or aluminum) used to collect, distribute, and transfer electrical power between incoming and outgoing lines in a substation.
Design considerations of busbars are Current Carrying Ca-pacity: Sized to handle the maximum load current without overheating (e.g., 630A, 1250A, or higher, depending on sub-station capacity). ShortCircuit Rating: Designed to withstand fault currents (e.g., 25kA for 3 seconds).
A bus coupler is a device or arrangement used to connect two or more busbars to enhance the exibility, reliability, and continuity of power supply. It plays a critical role in managing power ow and ensuring operational efciency.
-
Lightning arrester
A lightning arrester (or surge arrester) in a is a critical protective device designed to safeguard the substations electri-cal equipment, such as transformers, circuit breakers, busbars, other components, and working personnel from overvoltages caused by lightning strikes or switching surges.
-
Circuit Breakers
Circuit breaker is a protective electrical device designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and interrupt fault feeding.
-
Oil circuit breakers: It is used in the range of 33 to 132kV
-
Vacuum circuit breakers: It is used in the range of 22 to 66kV
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Air circuit breakers: It is used in the range of 132 to 765kV
-
SF6 circuit breakers: It is used in the range of 132 to 765kV
-
-
Isolator
An isolator, also known as a disconnect switch, is a me-chanical switch used in substations to ensure a complete disconnection of a part of the circuit from the power system for safe maintenance or service. As the name implies isolator
isolates one portion of circuit from another and is not intended to be opened while current is owing.
-
Instrument transformers
Instrument transformers in a 33/11 kV substation are critical fo measurement, protection, and control of electrical systems. They step down high voltages and currents to safe, standard-ized levels for meters, relays, and other devices. The various types of instrument transformers in substation are:
-
Current Transformer
-
Potential Transformer
-
-
Relay
A relay is an electrically operated switch. Many relays use an electromagnet to mechanically operate a switch, but other operating principles are also used, such as solid-state relays. Relays are used where it is necessary to control a circuit by a separate low-power signal, or where several circuits must be controlled by one signal.
-
Capacitor Banks
A capacitor bank is a grouping of several identical capaci-tors inter connected in parallel or in series with one another. Capacitor banks are used to improve power factor, enhance voltage stability, and optimize the efciency of the electrical distribution system.
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Power Transformer
Power Transformer is one of the most important equip-ments in a power transmission and distribution system. A power transformer is a xed device that converts power from one circuit to another without changing the frequency. A transformer is introduced as a static device because there are no moving or rotating components.
-
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SPECIFICATIONS OF EQUIPMENTS
The reliable operation of a 33/11kV substation mainly depends on the proper selection and specication of its equip-ment. Each component used in the substation is designed according to system voltage, load requirements, fault level, and operational safety standards. The major equipment used in the substation includes power transformers, current transform-ers (CTs), instrument voltage transformers (IVTs), lightning arresters, isolators, and vacuum circuit breakers (VCBs).
These devices perform important functions such as voltage transformation, protection, switching, measurement, and fault isolation. The technical specications of the equipment de-termine the efciency, stability, and reliability of the overall power system.Our work presents the detailed specications of various substation equipment used in the proposed 33/11kV substation design.
TABLE I
Specifications of Lightning Arresters for 33kV and 11kV Systems
TABLE V Specifications of Power Transformer
S. No.
Particulars
33kV
11kV
1.
Type
MOV
MOV
2.
Highest System Voltage
36kV
12kV
3.
Rated Voltage
30kV
9kV
4.
Nominal Discharge Current
10kA
5kA
5.
Maximum Continuous Over Voltage
25kV
8kV
6.
System Frequency
50Hz ± 5%
50Hz ± 5%
S. No.
Particulars
Ratings
1.
Make
Toshiba
2.
Rated Power
12.5MVA
3.
Rated Voltage
HV : 33kV
LV : 11kV
4.
No. Of Phases
3
5.
Tappings On
HV side
6.
Tapping Range
±5% in 2.5% steps
7.
Tap Changer
OLTC
8.
Vector Group
Dyn11
9.
Type Of Cooling
ONAN
10.
Percentage Impedance
7.49%
TABLE II
Specifications of Potential Transformer for 33kV and 11kV Systems
S. No.
Particulars
33kV
11kV
1.
Type
IVT
IVT
2.
Highest System Voltage
36kV
12kV
3.
Primary/Secondary Voltage
33 110
3 / 3
11 110
3 / 3
4.
Rated Burden
50VA
100VA
5.
Class Of Accuracy
0.2/3P
0.2/3P
6.
Power Frequency Withstand Voltage
70kV
28kV
7.
System Frequency
50Hz
50Hz
TABLE VI
S. No.
Particulars
33kV
11kV
1.
Type
Vacuum
Vacuum
2.
Highest System Voltage
36kV
12kV
3.
Short Circuit Current
25kA for 1 second
25kA for 1 second
4.
Rated Short Circuit Making Current
62.5kAp
62.5kAp
5.
Operated Mechanism
Motor/Man operated
Motor/Man operated
6.
Max. Break Time
60ms or less than 3 cycles
60ms or less than 3 cycles
7.
Max. Closing Time
150ms
150ms
8.
Pole
3 pole gang operated
3 pole gang operated
9.
System Frequency
50Hz
50Hz
Specifications of Vacuum Circuit Breaker for 33kV and 11kV Systems
TABLE III
Specifications of Isolator for 33kV and 11kV Systems
S. No.
Particulars
33kV
11kV
1.
Type
DB
DB
2.
Highest System Voltage
36kV
12kV
3.
Short Circuit Current
25kA for 1 second
13.1kA for 1 second
4.
Rated Peak Withstand Short Circuit Current
62.5kAp
33kAp
5.
Operated Mechanism
Motor/Man operated
Man operated
6.
Operating Time
12 second or less
12 second or less
7.
Pole
3 pole gang operated
3 pole gang operated
8.
System Frequency
50Hz
50Hz
-
DESIGN CALCULATIONS
-
Lightning Arrestor
For 33kV side
-
Highest System Voltage (H.S.V)
H.S.V = 1.1 × 33 = 36kV
Specifications of
TABLE IV
Current Transformer for
Systems
33KV
AND
11KV
-
Rated Voltage
Rated Voltage = Highest System Voltage×Ground Fault Factor
= 1.1 × 36 × 0.8 = 25kV
Ground Fault Factor = 0.8 (for effectively/solid grounded system)
-
Maximum Continuous Operating Voltage (MCOV)
table cellspacing=”0″>
S. No.
Particulars
33kV
11kV
1.
Highest System Voltage
36kV
12kV
2.
Primary/Secondary Current
1200-600-150/1-1-1-1 A
100-1200/1-1-1-1 A
3.
Short Circuit Current
25kA for 1 second
13.1kA for 1 second
4.
Burden
30VA
20VA
5.
Peak Short Circuit Current
62.5kAp
33kAp
6.
Operated Mechanism
Motor/Man operated
Motor/Man operated
7.
Max. Break Time
60ms or less than 3 cycles
60ms or less than 3 cycles
8.
Max. Closing Time
150ms
150ms
9.
Pole
3 pole gang operated
3 pole gang operated
10.
System Frequency
50Hz
50Hz
fcov × Maximum System Voltage
MCOV = 3
1.1 × 36
Where,
= 3 = 22.86kV
fcov = 1.1 for H.S.V > 100kV
fcov = 1.05 for H.S.V < 100kV
-
Continuous Operating Voltage (COV)
fcov × Nominal System Voltage
-
Turns Ratio
V2 = N2 V1 N1
110
COV =
=
3
×
1.05 33
3 = 20kV
=
33000
-
Highest System Voltage
= 36kV
-
-
Nominal Discharge Current
= 10kA
(As per IEC recommendations)
For 11KV Side
-
Highest System Voltage (H.S.V)
H.S.V = 1.1 × 11 = 12.1kV
-
Rated Voltage
-
Power Frequency Withstand Voltage
= 2.1 × 36
70kV
For 11KV Side
-
Rated Primary Voltage
V1 = 11000V
-
-
Rated Secondary Voltage
Rated Voltage = Highest System Voltage×Ground Fault Factor
= 1.1 × 12.1 × 0.8 = 10.64kV
Ground Fault Factor = 0.8 (for effectively/solid grounded system)
-
Maximum Continuous Operating Voltage (MCOV)
fcov × Maximum System Voltage
-
-
Turns Ratio
V2 = 110V
V2 = N2 V1 N1
110
=
11000
MCOV =
3
= 1.1 × 12.1 = 7.33kV
3
-
Highest System Voltage
= 12kV
Where,
fcov = 1.1 for H.S.V > 100kV
fcov = 1.05 for H.S.V < 100kV
-
Power Frequency Withstand Voltage
= 2.1 × 12
28kV
-
Continuous Operating Voltage (COV)
COV = fcov × Nominal System Voltage
3
1.05 × 11
C. Current Transformer
For 33KV Side
-
Rated Primary Current
31.5 × 106
= = 6.66kV 3
-
-
Nominal Discharge Current
= 5kA
(As per IEC recommendations)
-
-
-
-
Potential Transformer
For 33KV Side
-
Rated Primary Voltage
V1 = 33000V
I1 = 3 × 33 × 103
= 551.107A 600A
-
Rated Secondary Current
I2 = 1A
(As per IEC standards)
-
Selected Current Transformer
CT Ratio = 1200-600/1-1-1-1A
-
Turns Ratio
I N
2 = 1
-
Rated Secondary Voltage
V2 = 110V
I1 N2
1
=
600
-
Highest System Voltage
Vm = 36kV
-
Short Circuit Current
Isc = 25kA
-
Rated Short Circuit Making Current
-
Peak Short Circuit Current
Ip = 2.5 × 25
= 62.5kA
For 11KV Side
-
Type of Isolator
For 11KV Side
Im = 2.5 × 25
= 62.5kA
Double Break Isolator
-
Highest System Voltage
Vm = 12kV
-
Short Circuit Current
-
Rated Primary Current
I = 31.5 × 10
6
1
3 × 11 × 103
= 1653.32A 1200A
-
Rated Secondary Current
I2 = 1A
(As per IEC standards)
-
Selected Current Transformer
CT Ratio = 100-1200/1-1-1-1A
-
Turns Ratio
Isc = 13.1kA
-
Peak Short Circuit Current
Ip = 2.5 × 13.1
= 32.75kA
E. Circuit Breaker
For 33KV Side
-
Type of Circuit Breaker
Vacuum Circuit Breaker
-
Highest System Voltage
I2 = N1
V = 1.1 × 33
I1 N2
1
=
1200
m
= 36kV
-
Short Circuit Current
-
-
-
-
Highest System Voltage
Vm = 12kV
-
Short Circuit Current
Isc = 25kA
(As per IEC standards)
-
Making Current
Isc = 13.1kA
-
Peak Short Circuit Current
For 11KV Side
Ip = 2.5 × 25
= 62.5kA
D. Isolator
For 33KV Side
-
Type of Isolator
Ip = 2.5 × 13.1
= 32.75kA 33kA
Double Break Isolator
-
-
Type of Circuit Breaker
Vacuum Circuit Breaker
-
Highest System Voltage
Vm = 1.1 × 11
= 12.1kV
-
Short Circuit Current
-
Highest System Voltage
Vm = 36kV
-
Short Circuit Current
Isc = 25kA
Isc = 25kA
(As per IEC standards)
-
Making Current
-
-
-
-
-
Ip = 2.5 × 25
= 62.5kA
F. Power Transformer
-
Rated Primary Voltage
V1 = 33kV
-
Rated Secondary Voltage
V2 = 11kV
-
Full Load Current on Primary Side
MVAT × 106
References
-
Electrical Power Systems By C.L. Wadhwa
-
https://www.researchpublish.com/upload/book/DESIGND
-
International Electrotechnical Commission, IEC 60044 Instrument Transformer Standards.
-
Power System Engineering, Power System Engineering, Tata McGraw-Hill Education.
-
Bureau of Indian Standards, IS 3156 Current Transformers Specica-tions.
1
I1 = 3 × V × 103
×
12.5 106
= 3 × 33 × 103
= 218.6A
-
Full Load Current on Secondary Side
MVAT × 106
2
I2 = 3 × V × 103
×
12.5 106
= 3 × 11 × 103
= 656.0A
-
Short Circuit Current on Primary Side
Isc = 25kA
(As per IEC standards)
-
Short Circuit Current on Secondary Side
Isc = 10.5kA
(As per IEC standards)
-
-
-
Conclusion
This work provides an overview of diffrent equipment in 33/11KV substation and design considerations of substation equipment. Each element of the substation from power transformers, circuit breakers, and lightning arresters to control systems, auxiliary supplies, and protection schemes must be carefully selected and engineered to meet operational, safety, and future expansion needs. Key design considerations include load demand, short circuit levels, equipment ratings, system reliability, and compliance with national and international standards such as IEC and IS codes.
This work provides an understanding of
-
Single line diagram of 33/11KV substation
-
Various Substation equipment in detail
-
Specications of the different equipment
-
Design calculations of various substation equipment
