Ratings of Equipment Used in Substations

Ratings of Equipment Used in Substations

It is very important to know about the ratings of electrical appliances or equipment being used in a substation, otherwise, acquisition of the desired objectives as well as installation becomes difficult. Moreover, it becomes difficult to assess their price due to the unclear equipment ratings, as a result, it is not possible to make an accurate initial estimate of the costs going to be incurred on the construction of a substation. Ratings of essential equipment being used on any specific rating substation (125 KVA to 30 MVA) have been given below. Remember that substations are of different ratings or capacities and as such cost of electrical appliances being used in a substation also changes according to the substation rating. 

Power Transformer

A power transformer normally consists of the following ratings;

(a). Type of Transformer w.r.t Core

(i). Shell Type (ii). Core Type

(b). Types of Transformer w.r.t cooling 

1. Air Natural A. N
2. Air Blast A. B
3. Oil Natural Air Natural O.N.A. N
4. Oil Natural Air Forced O.N.A. F
5. Oil Forced Air Forced O.F.A. F
6. Oil Forced Water Forced O.F.W. F
  1. Number of phases

L.V. Side = 3-phase

H.V side = 3-phase

(d). Types w.r.t Voltage

(i). Step Down (ii). Step Up

(e). Rated Voltage Between Phases

(i). H.V Side = 132 / 220 KV

(ii). L.V Side = 11 KV

(f). Rated Frequency; 50 HZ

(g). Permissible Temperature

(i). Core Side Temperature; 50°C

(ii). Oil Temperature; 39°C

(h). Efficiency at 0.8 P. F

(i) At full load; 99%

(ii). At half load; 99.2%

(i). Rated Power; KVA/MVA

(j). Regulation at 0.8 P. F; 3.88%

In the following table, rating plate data of a power transformer has been given for explanation purposes;

Rate Plate Data of A Power Transformer

S. No Description of Data Required to Be Given Remarks For Example (e.g)
1 Kind of transformer Core type power transformer
2 Relevant standard, Year I.S …198 …
3 Manufacturer’s name
4 Manufacturer’s serial number 156252
5 Year of Manufacturer 1984
6 Number of phases 3
7 Rated power 1000 KVA
8 Rated frequency 50 HZ
9 Rated voltages HV 33 KV, STAR


10 Rated currents HV


11 Vector group symbol
12 % Impedance voltage; at rated current 8
13 % Reactance voltage %;
14 Types of cooling ONAN
15 Total Mass;

Mass or Insulation Oil

Transportation Mass

Untacking Mass





16 Insulating liquid (if not oil type)
17 Quantity of Oil (Liters);
18 Details about Tap-Changer;
19 Rated Insulation levels;

Power frequency withstand voltage;

Lighting Impulse Withstand Voltage;


KV rms

KV peak

Circuit Breaker

The circuit breakers of the following different ratings and types are used on the substations;

Types of C.B

  1. Air Circuit Breaker
  2. Oil Circuit Breaker
  3. Vacuum Circuit Breaker
  4. SulphurHexa Fluoride (SF6) Circuit Breaker

The identification of any circuit breaker is done through the following specifications;

Specifications / Ratings of Circuit Breaker

1 Type of medium for arc-extinction Air/Oil/Gas/Vacuum
2 Rated voltage between phase to phase
3 Rated breaking current 40KA
4 Types of construction Indoor metal clad type

Outdoor type, metal clad SF6

Gas insulated type

5 Type of operating mechanism Pneumatic, spring, hydraulic
6 Total break time 60ms
7 Structural form Live tank type (housed in porcelain). Dead tank type (housed in metallic tank at earth potential)
8 Rated continuous current 2000A
9 Reclosing time 300 MS
10 Rated frequency 50 HZ

The rating of SF6 circuit breaker being used on 132 KV, is as follows;

(i) Rated voltage 145 KV
(ii) Rated continuous current 2000A
(iii) Rated frequency 50 HZ
(iv) Short circuit making current 50 KA, rms
(v) Short circuit breaking current 31.5 KA, rms
(vi) Short time current (1 sec.) 31.5 KA
(vii) Total break time 60ms
(viii) Reclosing time 360ms
(ix) Impulse withstand voltage 650 KV Peak
(x) Power frequency test voltage

(1 min. 50HZ, rms, Dry)


Normally, porcelain-constructed (also constructed through glass and resin) under-noted insulators are used on the substations;

(i). pin-type (ii). Post type (iii). Suspension (iv). Strain (v). shell type (vi). Desc type

Apart from aforementioned the types, several other types of insulators are also used, however, the use of pin-type and desc-type insulators is the most common. The rating of both these types is as follows;

Rating of Pin Insulator

1 Working voltage 11 KV
2 Total creeping distance 230 mm to 320 mm
3 Protected creepage distance 110 mm to 140 mm
4 Cantilever strength 10KN
5 Height 63.5 mm to 168.285 mm
6 Maximum diameter 57.15 mm to 146.05 mm
7 Diameter of groove side 9.52 mm to 19.05 mm
8 Trop 15.885 mm to 19.05 mm
9 Spindle size/ diameter of shank 15.885 mm to 28.5 mm
10 Tensile strength 2040 Kg
11 Impulse withstand voltage 110 KV rms

Rating of Disc Insulator

1 Working voltage 11 KV to 33 KV
2 Disc diameter 152.4 mm to 255 mm
3 Total creepage distance 292 mm to 340 mm
4 Protected creepage distance 175 mm to 215 mm
5 Failing load min. 70 KN
6 Dry withstand voltage 70 to 75 KV rms
7 Wet withstand voltage 40 to 42 KV rms
8 Impulse withstand KV Peak 110
9 Unit weight 6 Kg
10 Unit spacing

145 mm


The types and ratings of the isolators used in a substation are as follows;

S. No. Type Rating
1 Horizontal break 7 KV
2 Vertical break 12 KV to 420 KV
3 Pantograph 220 KV to 420 KV and above

Reference Data For 132 KV Isolators

1 Rated voltage 132 KV
2 Highest system voltage 145 KV
3 Rated normal current 2000A
4 Short circuit for 1 sec. 31.5 KA
5 Rated insulation level for power frequency withstands test 1 min. 50 HZ 300 KV rms
6 Type of mechanism (i). Manual (ii). Pneumatic (iii). Spring operated motor wound
7 Construction (i). Horizontal

(ii). Vertical

(iii). Pantograph

8 Operating time for opening

20 sec.

Current Transformer

The current transformers (CTs) being used in the substations apart from current measurement are also used for protective purposes. For example, over current protection, earth fault protection, differential protection, impedance protection, etc. The rated characteristics of a current transformer, which are necessary to be known, are as follows. However, it ought to be remembered that the VA rating of an ordinary current transformer being used for three-phase tends to be 15 VA with its current ratio being 400 / 5 amperes. 

Rated characteristics of CTs

1 Rated primary current
2 Rated short time current (primary)
3 Rated secondary current an outdoor current
4 Rated exciting current
5 Rated burden
6 Current error or ratio error
7 Phase angle error
8 Composite error
9 Accuracy class
10 Over current factor
11 Insulation level

Some important ratings of the current transformer are as follows;

(1). Rated primary currents 0.5 – 2.5 – 5 – 10 – 12.15 – 15 – 20 – 25 – 30 – 40- 50 – 60 – 75 – 125 – 150 – 200 – 250 – 300 – 400 – 500 – 600 – 750 – 800 – 1000 – 1250 – 3000 – 4000 – 5000 – 6000- 7500 – 10000 Amp.
(2). Rated secondary currents 1- 2- 5 Amp.
(3). Rated output / Rated burden 2.5 – 5 – 7.5 – 10 – 15 – 30 VA
(4). Rated short time current with duration 750A for 0.5 sec., 525A for 1 sec., 300A for 3 sec.
(5). Current ratio

400 / 5 Amp.

Potential Transformer

The potential transformers or voltage transformers are also used for measurement and protection purposes in substations. The ratio and rating of a potential transformer for a 11 KV substation is as follows;

No. of Phases 3-Phase
VA Rating 100 VA
Ratio 11000/ 110 V

However, it is important to understand the following points before selecting a potential transformer. Remember that all these points deal with rating;

Rating of P.T

1 Rated primary voltage
2 Rated secondary voltage
3 Rated burden
4 Supply frequency
5 Number of phases
6 Class of accuracy
7 Insulation level
8 Types of construction
9 Rated transformation ratio
10 Ratio error

Voltage factor


The rating of a tap-changer being used for increasing or decreasing the voltage level alongside a power transformer is as follows;

Rating of Tap-Changer

1 Type of Tap-Changer OFF- Load or ON- Load
2 SI. Of Tap-changer
3 No. of Tsp Positions
4 No. of Tap Steps
5 Specifications IS …

Year …

6 Tapping Power for Each Tape
7 Extreme value of Voltages
8 Corresponding Values of Currents

Surge Arrester

To protect electrical appliances installed on a substation against high voltage surges, multiple different types and ratings surge arresters are used. The rating of a surge arrester being used at a 400 KV substation is as follows;

Ratings of surge Arrester for 400 KV Substation

1 Rated voltage of the surge arrester 360 KV rms
2 Nominal discharge current 10 KA Peak
3 Minimum discharge capacity 5K- Joules/ KV
4 Maximum switching surge sparkover voltage 850 K Peak
5 Maximum lightning impulse sparkover voltage 875 KV Peak
6 Pressure relief class

Class A

Key Diagram of 33 KV/11 KV Substation

Ratings of Equipment

Single Line Diagram of 132 KV/ 11 KV Substation

Ratings of Equipment

Standard symbols of Power System

Standard symbols of Power System Used in Single Line Diagram of Sub Station

Ratings of Equipment

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My name is Shahzada Fahad and I am an Electrical Engineer. I have been doing Job in UAE as a site engineer in an Electrical Construction Company. Currently, I am running my own YouTube channel "Electronic Clinic", and managing this Website. My Hobbies are * Watching Movies * Music * Martial Arts * Photography * Travelling * Make Sketches and so on...

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