Ratings of Equipment Used in Substations

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Table of Contents

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

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 LV 11 KV, DELTA
10	Rated currents	HV LV
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	Kg Kg Kg Kg
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

Air Circuit Breaker
Oil Circuit Breaker
Vacuum Circuit Breaker
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)

Insulators

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

Isolators

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
11	Voltage factor

Tap-Changer

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